This is quite possibly the best kids' martial arts program I've ever seen.
http://www.youtube.com/watch?v=HILkEZiK60E
http://www.youtube.com/watch?v=GVCzvTpn8z0
They can go all-out, and it's clearly enjoyable for them; just a more intelligent extension of the play-wrestling we all did with our siblings. In fact, I'd probably say that a person with 5-10 years of childhood experience in this would have more of an edge coming into aikido later than if they'd spent that time doing Kidz Lite Aikido (tm).
[Note: Usual disclaimer about being uncertain about this being the correct topic to post this under.]