YEAH FOR YOU DOING SOMETHING FOR YOURSELF! All moms should do this on occassion.
It is great their dad is willing to stay at your house. Have him come over prior and see what the routine is. Bedtimes, meals and so on need to stay on track so it lessens any stress.
Kids are really resilient. I know only because my ex moved away when my kids were 12 mos and 4. The first few weeks they didn't even notice a huge change as he travelled a lot.
I would go get $1 treats for them (Target has a great dollar aisle) for every day you are gone, have your hubby give them to them each morning as a gift from mommy. Call them before bed and say prayers,sing songs or whatever you typically do for them. If you cannot do it every night, do it at some point during the day.
Kids that young have no real sense of time, a day, three days, two weeks. So the time will fly by for them I am sure.
The biggest thing is you not to over do any talks about being gone, don't show them you are sad or stressed about it as they pick up on that. Keep it upbeat. The first day or two they may be off kilter but if your ex keeps up with their routine, baths and so on then I am sure they will be fine. Kudos for you all being able to do this and him willing to stay with them so you can go!! That shows such maturity in your relationship and will benefit the kids so much down the road.
Just show the 4 year old a calender, show her when you leave, when you get back with big red hearts to mark those days. The 18 mos old will really be the easiest as they cannot understand things like a 4 year old can.
I wouldn't have a long dialouge about leaving, but answer questions and tell them you are taking a "mommy vacation" and will be back soon. Have a blast, try not to worry and enjoy your time away! You will be a better mommy for it!