I have the same problem. I gained one pound in a week. I ate many healthy things with protien. That is your best friend. Also vitamin C can help out a lot. I hope that this helps it has helped me.
How Can A 12 Year Old Gain Weight Fast?
You can gain weight fast by not using any energy, sitting on the couch watching T.V. Or playing play station oh and eat lots of oodles of noodles, potatoes, french fries, rice, bread, ice cream, chips, cookies and fattening soda.
Go to your nearest health food store and ask for a delicious whey protein shake mix in the childs favorite flavor,and follow the routine instructions on the package,which will suggest that its taken with 3 full healthy meals.
Eat lots of fattening fried cheesy foods such as fried chicken nachos and many other foods
Eat bananas and cheese
I suggest him or her to find something heavy, like a big coil of wire THAT IS NOT SPIKY!!! I am 10 years old and it worked for me!! :)