why have I never thought of this?

These pants were made from the sleeves of a men's dress shirt.  I can't take credit for this--it was the idea of the woman who organizes the local Waldorf-inspired play group we go to.  She had been making adorable sundresses for her daughters from her husband's discarded shirts, and was able to turn the sleeves into matching pants/britches/bloomers.  She started to make these for her son, but they weren't going to be big enough, so now Joseph has new pants.

I haven't tried to make these myself (all I did on these was sew the waistband), but the general idea is that you detach the sleeves, put one sleeve inside the other (right sides together) and cut a crotch (use a pair of pants that fits your kid or just wing it).  The only seam you have to sew is the crotch seam.  Then you add an elastic waist, and you're done.  I'd say that a men's size medium shirt would make pants that fit a 2T or 3T, maybe even bigger.

On these pants the buttons are on the back of the leg, but you could do it the other way around..