Congratulations and good luck with the coming twins. I have to agree with most of the other responses that gifts are definitely not necessary.
In my opinion that's probably one of the things that has enabled all our GenXers and following generations in becoming young adults and teenagers who think everything is about them and not able to deal with losing (along with twelve valedictorians, everyone getting a trophy just for participating, etc.
On the other hand I think in general it is nice for the little ones to have somthing to keep them included in the fun. When my second child was born we had the older sibling help open the gifts and she was just as excited with the activity and some of the gifts (when you think of it the older sibling ends up playing with most of the items, too!)
If you still want to give a gift, the suggestions you received by the other moms are great, keep it small or try to convey to the little one that this is something for both of the children to share (something they'll definitely need to learn- :)