I have a 3 yo who has always been on the skinny side! Even now she and her sister who is 18months weigh around the same which is only 12kg!

I think at 12 months she was only about 7-8kg. I will look for her book later and have another look... She has always been healthy though so i have not stressed to much about it but was made to feel pretty crappy about her weight gain now and then from people and the mchn's.
About 12 months ago i took her to a paed and he just said she is a skinny kid and not to worry about it unless she starts to lose weight! She is ultra active so i put alot of it down to that too