Where tenants have a child while they are in occupation of the property, they would not be in breach of the contract or a maximum occupancy clause.
For example, if the maximum occupancy on the contract was 2 for a one bedroom flat, if the tenants had a child during the tenancy this would not put them in breach of the contract. It is always advisable that the landlord is notified, though.
Any term added to the contact which seeks to prevent tenants from having children would be found to be unfair and unenforceable. The landlord would not have grounds to end the tenancy on this basis and would only be able to end the tenancy via one of the usual routes.
Information on this site is by way of general guidance only and may not apply in your particular circumstances. You should not act or refrain from acting upon information on this site without seeking independent legal advice.