Here's what Cornell Labs says:
Cowbird eggs hatch faster than other species eggs, giving cowbird nestlings a head start in getting food from the parents. Young cowbirds also develop at a faster pace than their nest mates, and they sometimes toss out eggs and young nestlings or smother them in the bottom of the nest.
Nature can be so cruel. Do you think your daughter can bare witness to these possibilities? I have an 8 year old daughter too - I'm really not sure how she would cope with the death of cute little birds. Officially I don't think you're supposed to remove the egg since Cowbirds are a native species and Chipping Sparrows are not endangered, however it's something to consider. I suppose trying to incubate and raise the Cowbird separately would be too much work?
Best of luck!
-kris