I want to preface my answer by saying that I don't have twins, but having 2 kiddos and a third on the way, less is really more in our house...
I generally purchase savings bonds for the kids' parties that I/my children attend.
This usually means less toys, less pieces to loose, less clothes to hang/wash/etc. AND it is an investment in their future. They are 4!! They really don't know any better and there will be SO much else there. Your "gift" will hardly be missed, but when they are using that money for school, a house, a first car or some other big investment, they will be so happy.
Just my two cents on the topic.
~C.