You need to make ressies ahead for this and it will cost a little more per person, but you get reserved seating, guaranteed seating. There have been shows where the stand-bys do not get in. You have to secure your ressies with a credit card. They don't charge it; unless, you are a no show at Biergarten. I think you have up to 48 hours to cancel, if needed without being charged.
I've been to many of these CP performances and between waiting in line and the length of the show it's very hard for children in your age category to be patient enough to endure it all. Some parents do the following: one parent goes to the early show while the other entertains the children elsewhere. Then you switch parents and the other one can see the show if they want to do it. If you do this with a CP package you will have to explain the need for two different show performances. I'm not sure how that will work or not. Also, you can use this strategy for the wait time in the line by letting one parent go off with them and do other things until show time. Another tactic is being in the back of the outside theater; so, you can make an easy exit, if needed. It gets very dark at some points of the show and it's lighter in the back and easier to leave from there. Finally, it can get cold and children especially feel it. If not prepared for the cool nights, they can get very crabby and loud about it.
I don't want to make this sound terrible and some kids who have talent for singing, acting, etc. really enjoy the performance, but overall, if you ask them they would rather do other things at Disney at that age. If you have a family member in the choir, etc. sometimes that will hold their interest. If they were younger they might sleep through some of it, but your children's ages are against you for any sleep time there. If you have experience with long church services at Christmas, etc. and they do well there, it might be ok but this isn't church, either. Good Luck!