The problem is not with the children.
The problem is that this honeymoon is so far from being a shared experience of two people who love each other, it's really sad.
Your fiance is not your partner in this venture. He's not your partner in planning the trip, you feel forced to go, he's not involved with finding a week's worth of child care, he's dismissing your anxiety and fears.
You say you have no fears regarding the marriage - but I think maybe you do, or should. Is this the only time you have disagreed or where he has been controlling? Is this the only time he has tried to pry you away from family and friends and make you do what he wants?
Your mother not knowing where things are in town is not the problem. Two- and six-year-olds don't need much. A helpful neighbor, a map, and a well-stocked fridge should be fine. Her stamina with MS is an issue but do you have teens in the neighborhood who can come in after school and pitch in? They can help watch kids, throw in some laundry, bring in groceries?
But back to the honeymoon - this is all happening in a few weeks, he's booked the travel without talking to you, and is leaving ahead of time, and you two are so far from being on the same page, it's quite concerning. I'm wondering if that's what's got you all upset and anxious, and it doesn't have anything to do with the travel part.