So we've done BBB several times and the first time was when they were around your kids' ages. They loved it. Are you going to bring your own dresses or will you buy what the BBB sells? The dresses they sell are super cute but also super itchy. The last two times my girls did it, they had their own dresses which worked much better. The first time we did it, I didn't even know that you can bring your own. Also, depending on what hair style you go with, that could be an issue. My one daughter loved her hair but the braids were so tight that we had to take it all out the next day. Whatever you do, just make sure you stick to something that won't bother them or be really uncomfortable. We did BBB before a character meal so we had good pics for meet and greets. I'd think before a party, you could get some cool photos with all the special photo ops so that will be a nice perk.
Keep in mind that it will take a long time and they're not always on schedule so depending on what package you get, I'd suspect you'd be there for no less than 1.5-2 hours with wait time so it will cut into the early pre-party time. Also, make sure you've all eaten something before you go since it's not a short process and you can't eat inside of BBB. You could keep checking for openings that might work better for your schedule too as they do get cancellations.