There are numerous options. We've used some, and my kids kids have been invited to parties where the invitations have made some of these requests:
--please, no presents
--please, no presents, just your presence
--in lieu of gifts, we would appreciate a donation in Kayla's/Kierra's honor to a non-profit of your choice
----in lieu of gifts, we would appreciate a donation in Kayla's/Kierra's honor to [name a charity you like] or [name another cahrity you like] (and give the websites)
--We will be donating Kayla's/Kierra's gifts to CHOP/St. Christopher's/[name another charity]
As you said, your kids are really little, and they've been to birthday parties where they've seen kids receive gifts. They know the routine. If people actually bring gifts, it may be difficult for the birthday girl to part with them... If people don't bring gifts, that may be difficult, too!
So, perhaps have people bring gifts but let your daughter know in advance that she gets to choice a certain number of gifts (tell her how many) to donate to children who really need donations because they aren't as lucky as your daughter is, etc.
We have had family members make donations instead of gifts. For example, my parents have "adopted" zoo animals in honor of my children's birthdays. Maybe you can have some people contribute to this, and then your daughter can go to the zoo (the Philly zoo has a great Adoption Day, and Elwood Park has adoptions, too)and see "her" animal. One year my parents donated $ so my children could have their hand prints on a wall at their religious school. This was years ago, and tey still love seeing this. My kids also love theater, and my parents have given us theater tickets instead of toys. There is a lot of children's theater out there from which to choose!
Have fun! Happy birthday to your daughter!