Hand shape is very important!
It's important to keep fingers touching the keys all the time.
Fingers should be curved not straight
Fingers should be just in front of the black notes
Avoid droopy wrists!
It reduces mobility in the fingers and will cause problems later when new skills are introduced such as moving hands and finger crossings.
Finger 4 is often weak...
When pressing the 4th finger it often causes the other fingers to lift off. Especially if you press down hard.
If this is the case then press finger 4 much more lightly, this will help to keep the other fingers down without popping up
Keep your eyes on the music not your fingers!
Many children feel the need to look at their fingers instead of the music. This causes hesitations and slowness.
Try holding a magazine, book above your child's hands while they play.
I do it in lessons and it works well.