I've done a couple of week long trips to Rocky Mountain National Park, and yes it can get cold at night.   We always take extra blankets along and have good sleeping bags, but I doubt it gets below 45 this time of year.   Just be prepared.   Depends too on what kind of tent you have.    We have a tent that the screen under the fly can be closed so it keeps more heat in.    Never liked tents that have open screen under the fly, lets a lot of cold air in.   Have fun, the kids love it I bet.