Hi!! I have 5 year old twins! I would get art stuff. It is affordable and girls LOVE it. Plus that way you can make it a "share" type gift. Crayons, markers, finger paint, stickers, coloring books, paper, anything like that. I know my DD really loves those mosaic type art things. Where you stick these little jewels on a design. You can get affordable ones for about $6.
As far as a gift goes I usually spend $14 on my kids' friends birthday gifts and if the twins are invited I do buy a gift from each of them for about $14 each. However, being a parent of twins, I would never expect someone to spend $28 on them. I think if you spent about $20 that would be plenty fair. If you do the craft stuff you can get quite a bit for that much money and may be able to get away with even less because little things like stickers,paper, glue and markers always looks like a lot because the stuff is so affordable. I have even seen cute jars with feathers and goggly eyes and stuff in them for crafts. Those would be so fun for 5 year olds too! HTH and have fun at the party!