I suppose it depends on how much it would bother you to not be sitting together. I've learned that flights are short and it's not the end of the world if we're not sitting exactly where we want to be sitting. My kids are much younger than yours, and we ended up sitting apart on a Delta flight last year (flights changed so many times and we got separated in the changes). My seven year old was two rows up and across the aisle from me. DH and our five-year-old were together about 10 rows behind us. We managed, and it didn't affect our vacation.
If you check in at exactly 24 hours before your flight, you'll probably get B zone boarding passes, which should allow you to sit together (or at least in groups of 3 and 2). You might be closer to the back of the plane, but I haven't heard of people having trouble getting 2-3 seats together unless they're among the last to board.
That said, if the stress of not knowing your boarding zone is going to be high, that's what EBCI is for. Pay the money and rest assured.