Is it permissible to agree not to have children or to stipulate that in the marriage contract?

Firstly: 

We have discussed the ruling on misyar marriage [Misyar
marriage is where a man does a shar‘i marriage contract with a woman,
meeting the conditions of marriage, but the woman gives up some of her
rights such as accommodation, maintenance or the husband’s staying overnight
with her] in the answer to question no.
85369, where we stated that it
is a valid marriage if it fulfils the conditions and essential parts of
marriage and is free of any impediments, but it is not the best option
because of what it leads to of harm and negative consequences, that we
discussed there. 

Secondly: 

With regard to the couple agreeing not to have children, this
is either for a specific time period or it may be on a permanent basis. 

In the first case, which is where they agree not to have
children for a temporary period, there is nothing wrong with this. This has
been discussed previously in the answer to question no.
130395. 

But if the agreement between the spouses is never to have
children at all, then this is a matter concerning which the scholars
differed. Some say that it is permissible and some say that it is haram and
is not allowed. If this is made a condition in the marriage contract then
some scholars regard the contract as invalid and some think that it is valid
but the condition is invalid, and this is the correct view. Taking measures
to prevent having children at all is contrary to Islam, which encourages
people to have children multiply, and regards that as one of the aims of
marriage. However, that condition does not render the marriage contract
invalid; rather it is an invalid condition. If it is made one of the
conditions of the marriage contract, the contract is valid but this
condition is invalid and it is not permissible for either party to adhere to
it. 

Sharaf ad-Deen al-Hajjawi (may Allah have mercy on him)
said: 

If both or one of them stipulates a condition that there be
an option to cancel the marriage contract or the mahr, or that there is to
be no intimacy, or that he should bring the dowry at a certain time
otherwise there is no marriage, or it is stipulated that there is to be no
dowry or maintenance, or that this wife is to have less of a share of the
husband’s time than her co-wife, or a greater share, or he stipulates that
he will engage in coitus interruptus with her… The condition is invalid but
the marriage contract remains valid.

Al-Iqna‘ fi Fiqh al-Imam Ahmad ibn Hanbal,
3/193 

And Allah knows best.