YANBU - go ahead and do it, enjoy the holiday, enjoy the B-class comfort with DH. Meanwhile, the DCs will enjoy the in-flight entertainment. You'll be just a few rows in front of them and able to pop back briefly right after take-off. The crew will look after their refreshment needs, as they do for all other passengers.
I regularly fly long-haul with three 3 DCs, and we often have one who has to sit next to another passenger. I have never had any complaints, and some people appear to have enjoyed the company of an entertaining 13-year-old. If the plane has a 2-4-2 configuration and you can get them window and aisle seets, they won't even be next to strangers.
If your DH has to arrive fresh, then he has to be in B-class. Some of the people that I work with will only fly B-class on long haul just because to do anything else is much more tiring (they all work really hard).
It also depends on size. An economy class seat is not so small for 10 and 13 year olds, whereas if you (and DH) are tall or larger, you would benefit from the extra space.
Once you arrive, you can spend time all together.