Hi A.,
I also have an 18-month-old, no older kid. We have to eat out a lot since we travel a lot, so my son has a lot of experience of restaurants. I guess that perhaps yours is not so used to it, and gets uncomfortable or hyped-up, or both, and the misbehaviour is his way of communicating this. If my littlie wants to sit on my lap while he eats, then I let him - in my opinion, better this than a scene or having to leave, and it's not like he won't grow out of it. It's a little awkward but better than a meltdown. Not sure what to do about the older kid - I guess talk to her about what's expected and offer a reward for good behaviour, BEFORE going out? Toddlerhood is a tough stage for parents and kids. Good on you for sticking it out.
Oh, yeah, I forgot - if you are still breastfeeding, a nice feed before you go out does wonders: settles the kid and fills the tummy enough to last till the food arrives. If not, try a glass of milk and a very small cookie - good for both the bigger and the littler kid.