I'm a size 3 and have previously done a deep dive into the children's versions of adult shoes.
Most are not quite the same. There's a tiktok with someone comparing the Uggs adults to kids. The soles are thinner and slightly narrower I think.
With trainers the soles are often not as hard wearing. Remember kids shoes are designed for someone generally lighter I weight and are expected to be grown out of whereas adult shoes are expected to last longer.
I have some trainers/boots and wellies but I wouldn't get the kids uggs as I think they'd wear down really fast.