What kind of decorting will they let you do? if no table decor, can you string crepe paper or bday banners? I am not aware restaurants would allow this.

This Chinese restaurant's pretty flexible... they don't mind, especially we are in a private room.

I'd get a three piece lattice work like used in weddings. Put it behind the head table and decorate. That way the guest of honor will have a nice background for most of the pictures.

I like the lattice work idea. You could dress it up with silk plants/flowers, balloons, ribbons, or pictures from previous birthdays, etc. How about balloons and ribbons hanging from a ceiling light fixture?

I saw someone decorate a room for a child's birthday party once with a sea theme, which looked beautiful. They took over 100 balloons in sea colors, inflated them with helium (you can rent a tank from most party supply stores), tied them with sea colored ribbons and floated them to the ceiling. Using the ribbon handles, they pulled some of the ballons down and attached 3 colors of tuille (blue, white and seafoam green), ran these between some of the balloons to simulate waves. They cut fish and sea horse shapes out of foamcore board, painted/decorated them and taped/tacked them to the walls. A few of the cut-out shapes, they decorated both sides and used nylon thread to suspend them from the ceiling with a small tack. Maybe you should decide on a theme for your party first?

You could print a Happy Birthday banner using your computer and printer, to hang on one wall, or cut the letters out of foam core and hang those on the wall. You could even let the grandchildren paint or decorate the letters.

You can also just blow up some ballons and scatter them around on the floor. We,ve done that before. It was a very neat affect. They are light enough that they blow and not get in the way. Or we have gotten helium and blew them up and tied ribbon on them hanging down pretty long and let them rest on the ceiling(as long as its not popcorn celing).That looked pretty also.