We only 4 hours from Zion so we go every year as we enjoy it so much. We have
teenagers near your kids age and they have so much fun every year.
If you have 2 days I would only go to Zion. Both places will be hot, but it not
so hot you hate it. In Zion there places to cool down specially if you hike the narrows in the water.
There so much to do a see in Zion you will not be disappointed. If you like to hike, bike and 4X4 this is the place for your family.
I would recommend staying in Virgin, UT at the River Resort RV Park. Much much much better than the other places in Springdale, UT. You only a short drive or shuttle into the NPS gate.
If you limited on time I would recommend 2 different things for you.
1. If you have mountain bikes with you. Take them to the shuttle stop INSIDE the park pickup. They have racks on the shuttles you can put them on the shuttle. Go to the end of the route and bike downhill back to the shuttle starting point. The shuttles are not allow to pass you and thus there lots of
places to stop along the way. You get to see views you don't see on the shuttles.
2. Is take the shuttle to the end stop and hike into the Narrows. It amazing and cooling trip. If you like I can post a few pictures for you. You can hike a short or long as you wish. Just depend on the water flow on how wet you wish
to get. If you going do a long trip you need to consider to rent the insulated booties in springdale.
I can post some pictures if you like to see some of the amazing views of Zion.
Have Fun.