For kids, ideally, you want a 3D printer with an enclosed build area. The reason being that there are lots of hot parts. The hot end gets to around 200C, that's not very child friendly.

The other thing is it needs to be simple to use. 3D printing can still involve a lot of calibration and tweaking print settings to get the right result so what you want is a printer that just has a plug'n play set up, just press the button and go. In this respect you are probably better off getting a printer that uses it's own brand filament. This can be more costly for supplies but the advantage is that the printer settings are already set up to use their filament, no tweaking necessary.

With respect to these caveats I would suggest XYZ printing Da Vinci Junior 3D printers. These are specifically designed for children.