my solution to this (with my stepson when he was younger and for the future with our little boys) is NOT buying the kits but buying used Lego in bulk on eBay. That way they can go wild with their imagination, can follow the books to create but still have to substitute for missing parts and even if they want to keeps some of their creations, they have a lot of other blocks to build with. I am planning on switching DS4.5 from Duplo blocks to regular Legos when he turns 5 in the summer. We still use Duplos mainly because DS3 still puts things in his mouth so we try to only have bigger pieces of "stuff" around him. I have already started buying them on eBay (got 3 lots in the last 3 weeks totaling about 25lbs of lego pieces for about $115)
The largest quantity of Legos we have came from a garage sale. We got lucky and bought an 18qt tub that was over 3/4 full of assorted Legos for $5.00. I am indeed frugal and I love a good bargain!
