I am one of those evil landlords.
I do rent to people with children. One of my flats is in the same street as an outstanding primary so renting to people with children works really well for me as the flat never has any void periods.
I have had one really bad lot of tenants with two children, a toddler and a pre school one. They didn't pay rent and absolutely trashed the place. It cost me about £12k all in all to put right. The had no shame and on eviction day the bailiff had to get the police to carry them out in front of their young children.
It was a nightmare but not because of children because they were just the sort of folk who didn't like to pay and were self entitled.
The courts granted the eviction order at the first session despite children given the level of outstanding rental arrears (4 months in arrears guarantees a landlord eviction as long as you have served S21).
But the whole process took 7 months as I had to wait and serve S21 after 8 weeks nonpayment then wait for the court and the wait for the bailiff to have a slot for the eviction.
I nearly missed the court date as I had an EMCS with DD three days before. And I had to grant DH power of attorney to attend in my place. Stressful. And nightmare tenants don't consider the stress they put on others with their behaviour.
I currently have a couple with 3 children in the flat. So far so good and they seem ok. I expect there will be quite a lot of wear and tear afterwards with three young children but that is fine as long as they pay the rent, are nice to the neighbour and don't burn the place down.
Others here are correct. Leasehold property can be subject to clauses that include no pets and children (and wooden floors etc). And it's not always safe to rent a flat to those with children if it's on upper floors particularly old Victoria conversion with very large sash windows that are easy to fall out of.