Trek MT60. Very adjustable (bars and cranks). Not too heavy. Good off-road. We had one and it was still rideable by Son1 at 10yo.
Specialized Hot Rock, Kona Makena, Scott Scale Jr.
All the big companies offer good quality AL framed bikes now. There are some cheaper good bikes from Ridgeway and Giant too. Look for adjustable metal brake levers, gripshift and an Al frame. Front suspension is de rigueur, but superfluous. Colour is probably the biggest factor for a 5yo. White and Silver is asexual so has a wider appeal 😉
I’ve taught children to ride Swinley singletrack on many of the above.
Islabikes rightly have their fans. But they are a niche player with good product diversification. If I was looking again, I’d go for a used Trek MT-60 for one child, new if handing down.