Islabikes is always a good start. More expensive than most, but my lad got,his when he was 5 and the “trails came alive” for him. Okay maybe not, but he really loved it and it helped his confidence, was doing blues at Llandegla on it when he was 6. Passed down to his sister and she is doing the same. He went onto a Genesis Core 24 after (7/8) that and is still on it now, aged 9. Islabikes didn’t do anything with 24′ wheels with a fork,and disc brakes at the time, but they do now, but would probably still buy the genesis as it’s a bit cheap and has a triple front.