Wizkids make some great models to use in games and I know in the past there different clix games were very popular with the preteen and teenage kids who definitely couldn't afford mainstream wargames and many adults. Now of course the ones I will be showing they no longer make but can easily be bought. The mechwarrior line they did. I picked up 150 different mechs, vehicles and other things for like $20 a couple of months ago.
Now many of these games are great for kids you have in the preteen to teen range because they are easy to play and often in themes they find popular, superheros, startrek, dota 2 and others. But with my kid the mechwarrior line was the way to go.
These models we just use the simple d6 idea atm that he uses for DW and FSA. In the future we will use the actual rules. The main benefit of the clix range is you can often buy them second hand for rather cheap amount just by browsing ebay. You will also find expensive lots when people try to sell them like actual wargaming models or MTG stuff. The only problem is some of the ranges disappear rather quickly. As for rules they can often be found for free. I have yet to try any of the rules but will let you know when I do how well I think they will work for the under 10 range.
Wizkids current range can be found here.