T.B.
I think this is a great opportunity, but understand you not wanting to be away from the girls for that long. Is there a special reason he wants you to join him in July? Will the opportunity not be available another month? Is he always gone 10 days at a time? Maybe there will be another chance for you to join him on a shorter trip.
My kids would welcome the break from us for 10 days, but they're not 3 and 4 anymore. :) Would *you* be able to relax and enjoy the vacation if you were to go? If you don't think so, then I would say it isn't worth it.
Would you need to keep the girls at one place for the entire 10 days? Could you break it up between friends and family? Maybe the change of scenery would have them focused on enjoying the differences as opposed to missing Mommy and Daddy.
Good luck with your decision. Can't wait to hear what you decide!