LOL -- GIMP confuses me even without Easy Debian.

I can't speak for "best", but my kids really enjoy TuxPaint (or "neh-neh-neh" as my two-year old calls it; you'll understand if you run it with sound on). I'm using version 0.9.15b-1, which I believe is in Extras-Devel. If I recall correctly, the one in Extras is buggy (ironically). I don't have stamps, alas.

BTW, I just tried MyPaint for the first time (I had to get it the deb from the author's website -- I couldn't find it on Maemo downloads), and it seems like it is not sensitive enough. I have a screen protector on my N810, and I was trying to draw & it wasn't displaying anything. Finally, I pressed harder & it displayed what I drew. It seemed like I had to press too hard. It seemed slow, also.