I totally get the clothes thing my 1 year old weighs 9.3kg and size 0 is far too short but size 1 falls off her waist.
My daughter is allergic to egg, milk, soy and peanut so her diet is really restricted. She pretty much lives on fruit veg and meat. Chicken liver is good for iron, I mash it with pumpkin, kumara potato etc. and she feeds herself with a spoon. Also mince patties, I use orgran egg substitute to bind them, and bolonese sauce with mince. finger size bits of roast meats. I also have some great recipes for muffins, cakes and biscuits that are milk egg soy and peanut free. My email address is firstname.lastname@example.org. If you want the recipes let me know and I'll email them to you.
PS. 12kg is average for 2 years old, but my daughters weight projection is that she'll be just under 11 kg at 2 years. She had fail to thrive and her allergies so hasn't gained weight properly - she was 4070g at birth and it took her 10 mths to double that which should happen around 6 mths. She is in th 97 percentle for height (80cm). Don't worry about the weight as long as it is increasing not decreasing or staying the same.
Mum to Maya Grace 02-03, Sienna & Mercedes 10-06