I think they’re a great idea, but I can’t help thinking that less fat may be better for a kid and still offer sufficient “float”.
A wide rim with a 20″ (or is it 19″) trials tyre would probably give comparable float for the light weight of a kid as a full fat for an adult.
The full fat kids bikes are going to be more tolerant of not keeping tyre pressures checked which I always found to be the biggest hassle with kids. They’ll get on and ride a soft tyre and wreck the tyre and tubes, then the bike gets stuck in the shed and ignored.
But no matter, there’s going to be some kids growing up with great cycling muscles. 🙂