Nuts (almonds, macadamia's, etc)
Avocardo
Cheese
Yogurt
Dried fruit
olives
dense grainy bread
homade bananna iceblocks (although probably the wrong season for those).
Also anything with protien is good for building up weight healthily (if you can get them to eat it)

I also used to bake special cakes and slices that were choc full of vege, but also loaded with kjs, as a special treat - good thing is if you bake things yourself you can control what is going into them.


Also, with my DS1 (who is the skinniest little boy and always has been), we found that introducing a multivitimin really helped his situation. He no longer gets sick as often, and thus doesn't loose weight when he can least afford to. Unfortunately most multivitimins cannot be introduced until after the age of 2 - unless with doctors direction. But there are some good drink flavours on the market which contain alot of vitimins and minerals specifically designed for toddlers. We actually had to take DS1 off cows milk and put him on formula until he was old enough for a multivitimin.