It might be cheeper and easier to raise the floor. I'm sure a carpenter could make something that would fit exactly around an existing adult loo and you could cover it in lino/rubber matting to prevent escaped wee from soaking through it.
It would obviously be cheaper not cheeper. A wooden frame might make a bit of a noise, but it's likely to be more of a squeak than a cheep.
Children's loos make me think of visiting Amsterdam Zoo where they had family cubicles with a big loo and a little loo. DS was thrilled, "Look Mummy, you're doing a wee out of your bum and I'm doing a poo out of my bum and we're together."