At the time when my kids were little my girl friends and I would plan and work our visitation schedules so all the ex-husbands had the kids on the same weekends. We all met post divorce so the exes didn't know each other. So we had some fun girl time. We could go out dancing and then to breakfast and sleep in all weekend. We could arrange shopping trips or a weekend away and the exes were none the wiser.
You will be child free for a few days plan a get away for you and a girlfriend or just plan some not so kid friendly activities just for you. Young children would not enjoy and leisurely stroll through a museum or art gallery, you could go with a friend and then go out to lunch and just relax. Remember lunching with a friend in your pre-child days when you could talk and laugh and not have to wipe up spills, constant runs to the restroom, fielding questions (Mommy why does that man look so funny?). Window shopping without a child pulling you along to go look at bugs and dog doo doo.
The kids will be fine, even the most overbearing MIL can be a doting grandma. My daughter will tell her son just 3 night-nights and you will come home. Just keep your cell phone battery charged so he can call you. And don't worry about the ex; he needs to learn how to be a single dad, if he is across the country he will either sink or swim. If he starts to sink he will learn to swim.
Start making a list of all the things you want to pack for the kids and as you start to pack them check them off the list. Keep small things like binkys, tooth brushes, tooth paste, ect in ziploc bags. I would get small sample bottles or use travel bottles of the same shampoo and bath soap they currently use. You don't want them breaking out in a rash in case grandma buys brand x baby shampoo and you use brand w. Explain to hubby that the change in the water and maybe laundry detergent and fabric softner, if she uses it, will be enough change for children so young. If you have a LO that loves his glo-worm or a certain CD or DVD or night light, or blanket send it along. The kids will be with family but there will be so many differences that as adults we don't notice as much. Traffic noises, maybe a thunder storm, crickets chriping, ect will all be different and enough to keep them awake or scare them. The more familiar comfort things they have with them the easier they will settle in.
Don't worry they and you will survive and even have fun.