The concept of robotic armor, or suits, has become quite popular among teen fans of Lego and adult fans of Lego, and could suitably be adapted to the audience of younger children. Inspiration varies from various films, such as the power loader suit from the film Aliens, or from various anime, such as Ghost in the Shell or Mobile Suit Gundam. The most entertaining and fun features will be the articulation and the ability for the suit to fit a minifigure, as compact or comfortably as possible. Thus the term, HARDsuit.
These suits could vary from many different designs, these just being examples of the wide variety of piloted robotic suits that could be used. (The above design is not a military exosuit, but a mining suit with drill, the suits I speak of have more utilitarian uses, such as loading cargo, maintenance, etc.) Each design could be affordable enough, having larger hardsuits being pricier, due to the amount of parts and size.