My son has a snow suit that he wears when playing outside in the mud and stuff but the seams are wearing out between his knees. Does anyone have any good idead for some sort of tape (maybe iron on) that we can use to toughen up the inside of the legs that will deal with the friction, dirt and water?
(Gaffa tape has been rejected by the boss:-))

Maybe get hold of some Mcnett Seamgrip, it's a very tough, urethane-based seam sealer/adhesive. I've just used it to do a very similar job on some windstopper shorts where the seam was wearing and it's worked really well. In the past I've used it to reinforce a rucksack strap which was getting abraded by the pick of an ice axe when I carried it between the pack and my back and it managed that too. Good stuff.