Which of them should be given precedence: having children or doing the obligatory Hajj?

Firstly: 

Whoever is able to do Hajj, both physically and financially,
must hasten to do Hajj and it is not permissible for him to delay it. This
has been explained in fatwa no. 41702. 

Secondly: 

Having children is one of the basic aims of marriage, which
leads to many benefits. The Messenger (blessings and peace of Allah be upon
him) encouraged us to have children in more than one hadith. 

So what we advise you to do is to hasten to have children,
and do not delay it for any longer than you have already done, as you stated
that you delayed it for two years. Children are one of the greatest
blessings that Allah can bestow upon a person. 

Although it is obligatory to do Hajj right away – as
explained above – it is permissible to delay it if there is a valid reason
for doing so. You do not know when Allah may bestow upon you the blessing of
children. Allah may bless you with children and that may not be an obstacle
to your doing Hajj if Allah makes a means of doing it easily available to
you. 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was
asked: A woman is asking: I want to do the obligatory Hajj for the first
time. I am married and I have small children, the youngest of whom is five
months old, and I am breastfeeding, but the baby is able to eat other food
besides milk. My husband did not let me go for Hajj on the grounds that I am
still breastfeeding, and I do not want to take the baby with me because I am
afraid that she may get sick and be affected by the change in climate.
Moreover, she would distract me and take up my time. Is this one of the
things that make it permissible for me not to do Hajj this year? 

He replied:

There is nothing wrong with this woman who is in this
situation delaying her Hajj until another year, firstly because many
scholars say that it is not obligatory to do Hajj immediately, and it is
permissible for a person to delay it even if he is able to do it. Secondly,
this woman needs to stay and look after her children, and looking after her
children is a great good deed. The Prophet (blessings and peace of Allah be
upon him) said: “The woman is the shepherd of her husband’s household and is
responsible for her flock.” So I say: She should wait until next year, and
we ask Allah to make things easy for her and decree what is good for her.

End quote from Majmoo‘ Fataawa wa Rasaa’il al-‘Uthaymeen
(21/66). 

There is another reason for hastening to have children, and
not delaying it any longer, which is your wife’s desire to have children.
Having children is the right of both spouses. If one of them wants that, the
other does not have the right to refuse or delay it, unless there is a valid
excuse for doing so. 

With regard to Hajj, even though it is undoubtedly
obligatory, it is only obligatory for the one who is able to do it. If she
becomes pregnant and has children, that will be an impediment to doing Hajj.
It should be understood that striving to become able to do it or to meet the
conditions that make Hajj obligatory is not something that is required of
the accountable individual. So it is not obligatory for him, if he is poor,
to go and accumulate money that will make him financially able to do Hajj. 

If he is sick, it is not obligatory for him to seek medical
treatment for the sake of Hajj and take measures to improve his health so
that he will be physically able to do it, and so on. 

The conditions of being able to do it. and so on. come under
the heading of that without which an obligatory duty cannot be done. What
this means is that the obligation to do the act of worship is not
established in an individual’s case unless these conditions are met, and the
scholars stated that meeting these conditions is not obligatory for the
accountable individual. 

Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him)
said: This issue is referred to by the phrase: Whatever is essential to the
performance of an obligatory duty is also obligatory. 

Some people made a mistake concerning that, and divided it
into:

·       
that which the accountable
individual is not able to attain, such as good health in his limbs, and the
minimum number of people required for Jumu‘ah to be valid, and similar
things that he is not able to attain,

·       
and that which he is able to
attain, such as travelling the distance to go for Hajj, washing part of the
head when doing wudoo’, refraining from eating for part of the night when
fasting, and so on. 

And they said: That without which an obligatory action cannot
be done, and which the accountable individual is able to do, is also
obligatory. 

This categorisation is incorrect, because the things that
they mentioned are conditions of it being obligatory, without which it does
not become obligatory. Whatever is essential to something becoming
obligatory, the individual is not obliged to strive (to meet these
conditions), according to Muslim consensus, whether he is able to do it or
not, such as having the ability to do Hajj, or earning enough wealth to meet
the minimum threshold at which zakaah becomes due. If a person is able to do
Hajj, he must do Hajj, and if he owns enough wealth to meet the minimum
threshold for zakaah, then zakaah becomes obligatory for him. It does not
become obligatory unless these conditions are met, so he does not have to
make himself become able to do Hajj or to own wealth that meets the minimum
threshold. 

Therefore, those who say that being able to do Hajj means
(already) possessing sufficient wealth, as is the view of Abu Haneefah, ash-Shafaa‘i
and Ahmad, do not say that it is obligatory for him to strive to earn that
wealth. 

End quote from Dar’ Ta‘aarud al-‘Aql wa’n-Naql
(1/211-212). See also: al-Musawwadah fi Usool al-Fiqh (1/52) 

Ibn Muflih (may Allah have mercy on him) said: 

That which is essential to doing an obligatory duty is not
obligatory according to consensus, whether the accountable person is able to
fulfil that condition, such as striving to earn sufficient money to go for
Hajj and offer expiation… or not [?], such as the presence of the imam and
the quorum for Jumu‘ah prayer. 

End quote from Usool al-Fiqh by Ibn Muflih (1/211)

And Allah knows best..