My two, eight and nine, go everywhere including school everyday, on racing BMX s. It’s worked out really well, they absolutely love them and the lightness makes most things possible.
Not great in slop as the thinner tyres don’t have deep grip, but they fly every where else.
Got the first one two years ago this month for my younger boys sixth birthday. He (and she) loved it, and the daughter wanted one for her eighth birthday the following Jan. Haven’t looked back.
Sizes are called micro, mini, junior, expert, expert XL, pro in ascending order. The top tubes get longer, handlebars higher, and wheels thicker, but stsndover stays low. You can get longer seatposts for off the track.
Mini was about right for the lad at 6, daughter started on a junior at eight and has just moved up to expert after growth spurt, the junior is now about right for the lad but he loves his bike so much he’s stayed on it. Haro and gt are good makes. They now race and spend most rides shouting “squish – pop!” At me trying to teach me to jump.