Mushroom Tower Defense
I would almost say that this is a tower defense game for kids, but the stupid thing is that I've played it myself and had fun with it, though I would not count myself as a child anymore. At first I didn't know what the 'towers' were, but it were mushrooms which are available starting at 100 in the menu. The upgrade is based on different elements you can add to the individual towers, depending on the upgrade level of the tower they can be linked together with various elements. You also get some power-ups in between.