I'll probably be criticised for this, because I know that most 'serious' cyclists look down on cheaper bikes. However there is nothing fundamentally wrong with the kids' bikes from places like Halfords, Decathlon, etc.
Yes, they can be heavy. Apart from that, their components are usually okay and the bikes perfectly serviceable if they are set up and maintained properly. These two latter requirements are usually where parents let their children down, unfortunately.
My 9 year old son is quite happy riding his (second hand) 18 speed Apollo 24" wheeled MTB. I changed the tyres to semi-slicks so he can get about on tarmac more easily, and fitted a rear rack. Mudguards are worn outside the dry summer weather too. He can ride 20 miles quite easily (even with panniers) as long as there are not too many hills...
My daughter has just turned 5 and her hand-me-down (from a cousin) Apollo 'Cutie' 12" wheeled bike has been retired for a new, somewhat larger 14" wheeled 'Princess' bike from (wait for it) Toys'R'Us. We didn't buy it, it was given as present. Still single speed, a bit lighter, two brakes instead of one and riser handlebars. I won't be expecting her to ride 20 miles on it but it's fine for riding (on the pavement) to and around the local park, to and from Fringe shows, etc.
Lighter bikes would of course be nice, but relatively heavy ones have not stopped our kids from enjoying cycling. They are both in their own ways confident riders, know the basics of maintenance and repairs and my daughter has never used stabilisers, having ridden a wooden balance bike for a few years.
I suppose my point is that if kids want to cycle, they will regardless of the bike (as long as it functions). Having parents who cycle a lot is probably the most important factor, so that for them cycling is 'normal'. Then the bikes need to be set up correctly and maintained regularly. That's it. Good brands and lightweight materials are nice to have, but not essential in my book.