I use Photoshop but it's pretty spendy. There are some alternatives, though:
Paint Net is a little more full featured than ms paint, but a little limited, and the price is right since it's free.
A little more powerful is
Paint Shop Pro, which has free trials but will cost you about 50 bucks. Note that the
PC download version costs twice as much a disc because
lol, feth you, that's why.
Gimp is totally free and is quite powerful, but it's got a somewhat poor UI which makes it a little harder to pick up. It's pretty close to Photoshop in capabilities.
As a side note, instead of scanning in her doodles, you might want to look into maybe getting a Wacom or similar tablet. You're halfway there, and they're pretty cheap - $70ish new and I bet you can get them cheap on ebay.