I had my twins (now 5) first birthday at their grandmother house & I had no ideas then either. If I could do it again, I would pick bright colors as the theme. They are not old enough to pick one themselves, but is is a scientific fact that kids like things in bright colors. So just pick bright blue's, green's yellow's, pink's and soooo on. You could also have little activity centers for the older children. Your twins could also do them, use the Elmers paint-less finger paint, it's a pounder and works really good. There are also so many mess free finger paint out there, it's so much fun to watch them all.
Remember, they are not going to remember this party, so do what is fun for the older children & take lot's of pictures. Just wait for the 4th party to really go all out. That is the first year they really get into it.