If people can say 'no pets' I think it's perfectly reasonable to say 'no children'. My cat has never scribbled on the wall, peed on my carpets or smooshed poo into the gaps between the floor boards! It's definitely worth asking if the landlord would make an exception though. My mum rents out our old family house and allows children but the agents gave her their standard description to agree to and it said no pets. She queried it as it's a 5 bed family house, there are bound to be pets! The agent wasn't keen to change it but she made them. I bet most people just don't bother but probably would be open to it, for kids as well as pets.