Underweight toddler - what to do?

So after dd's 18 month check tomorrow she is way down the bottom of the scale for weight, but on 75th for height and head circ. I'm not the slightest bit worried, she is such a busy little girl and her gross motor skills are the equivalent to a 3 year old apparently so clearly her development is great. The problem seems to be she is too busy to sit down and eat. Any advice? The nurse suggested high protein but I don't know about that when she is so young? We eat healthy, i just have no idea how to get more food into her!!

I let DS graze and eat on the run, he also doesn't like to sit still! I used to hate my eldest 2 walking around with food but DS3 just doesn't stop so I gave in and let him eat while he's on the move.
I wouldn't like to go high protein either, maybe just a little more dairy if she likes it she can munch on a piece of cheese while she's on the go, maybe extra milk in a sippy cup. Dairy would help with her weight if you want her to put on a bit but if you're not worried I'd keep doing what you're doing :)