There really is not much that's decent when it comes to 20" kids forks.  Most are heavy, oversprung and crappy performing el-cheapo forks. There are, however, a few good options:

There are many 26" forks out there but there is virtually nothing in the 20" size that is both good and cost effective for use on a kids MTB.

There are very few air sprung forks for 24" wheels:

Weighs 295g, comfortable for small backsides, ballistic nylon protection around the back.

This is just a quick round-up of tyres in the 20" x 1 3/8 size (451mm ERD) that may be suitable for kid's use for offroad riding (not BMX track racing).

The rims