I've coached a bunch of kids and one challenge with the really small kids (with less developed muscles) is that some don't have the strength to clip in or clip out. I've found some pedal systems just don't work well, however good old-fashioned SPDs (or Ritchey or Wellgo) are great because you can really loosen up the tension.

Now, from a fear and skill perspective, I've found most kids do much better than adults when learning how to use clipless pedals.