Some people don't want kids ore maybe can't have kids or maybe have a painful memory from kids and if they have live in close contact with kids it may not be what they want.

They own the appartment, surely they can chose a little who comes in ? Is not "a house and Englishmans' castle ?" I think the same applies here and if the landlord doesn't want kids around, well so be it.

Move on, find somewhere else !