I was going to say the same thing about k'nex- it takes a fair amount of strength to snap some of the pieces together (and take them apart). They are great, though. We have especially liked a few sets with motors. I think they lend themselves more towards engineering-type building, with moving parts, and less so towards architecture, but they got tons of use here. One nice thing is that it is easier to build large things with k'nex than with lego, just due to the nature of the pieces. Sorry, no experience with magformers.
Regarding motors, you might consider something like a lego train (pricey, though). Ours was repurposed and rebuilt frequently and saw much more use than expected. We also use lego technic pieces a lot, some with motors, many with gears and joints and other moveable parts (again, they can be expensive, we happened to inherit a lot of them).
None of these are particularly geared towards architecture, though- not much help in that area, unless you are thinking of things like the lego architecture series. My kids thought these look cool, but are turned off by the building-specific pieces- looks like sets that would be built once and displayed, which is not their thing. My DD is finally getting her architecture fix in an elective at school, with some great 3d software programs, but they are impractical for home use (at least for us!).
