I love the idea of a book party. You can phrase it in a way that it won't sound like a tacky request. My family's mantra is that you never can have too many books. My kids always loved and still love getting books.
Just remember that children your kids' ages, especially Emma's age, love receiving presents, so if you do something other than tangible gifts, the kids may feel a loss. If you ask for all donations, your kids’ disappointment might overshadow whatever good feelings they might otherwise have about helping people with their donations!
Different things that my kids and/or their friends have done over the years, starting when my kids were about 3 and 6, or 4 and 7:
--A donation to a special fundraiser at our congregation, where hand prints were cast, and now their names and hand prints are on a special wall. (This was from grandparents.) If your congregation or future schools or neighborhood playground are having fundraisers where the kids can feel a part of them, this may be something to look into, especially if there is something tangible for them to view after-the-fact.
--Theater tickets. This is one of my kids' favorite gifts to receive. Again, this has been a grandparent gift. For young kids, this can be a Blue’s Clues or Dora show, etc.
--Donation to a charity with some meaning for the kids...if you take them to a soup kitchen or to help at a place that stocks food for the less fortunate, etc., then that could become a pet charity for them and the donation will mean something
--Adopt a pet at the zoo or aquarium. We've adopted pets for years, or have had family members do the same. This would be pricey for one friend to give, but family or a few friends going into it together might work
--As they get older, and depending on their interests, tickets to other types of things, like sports games, orchestra concerts for kids, American Girl Place, or the more local American Girl Fashion Shows, etc.
--Any other type of special event or place they might like to visit, like a trip to the Battleship NJ or the Please Touch Museum with their best friends, just as an example, or a trip to another town or city for a fun meal.
--Donate a certain number of their presents to a local charity that service families. If there are 20 gifts, for example, have the kids designate, in advance, how many to give away.
Have fun with it! Good luck!