About the weight..... My Dd is 2 and only weighs just over 10kg! Nothing to worry about there :-)
You can buy polydule from the chemist and sprinkle it in her food or if she is having any cows milk it can go in there... They use it with the elderly to keep them well nourished. It doesn't replace a healthy diet but it helps keep some weight on them.
This was recommended to me by my mchn from a dietician she spoke to when my son (who is now 4 yrs) was just 12 months old and fell right off the graph for his weight :-0. Instead of judging she sought some advice from a dietician on my behalf which was lovely!
I would def start by having a 12 month check tho and voice any concerns you have and go from there :-)