Guess it depends how old - e.g. our son had a CX/road bike when he was 9-10 that had 26” wheels that fitted fine on an old Elite wheel-on trainer I had.
He also learned to ride my rollers - I have some Elite Arian rollers that had enough adjustability for his small CX bike.
But for younger kids bikes with 18 or 20 inch wheels, I guess a regular trainer may not work…