I may be missing something, but if you know you will only have summer childcare expenses, and those expenses will be for example, $1,200. Can't you just set aside the $100 a month, then submit for reimbursement at the end of August, get the $800 back, and then get $100 a month for Sep-Dec.?
Logically, yeah, that makes sense.
In reality, though, most people aren't 'specially good at long-term planning, so they don't make the appropriate choice a year ahead of time -- don't even think about it. And so many people genuinely believe
they have no option but to live paycheck to paycheck, they figure they don't have the $100/month to spare.
However, I'll give the "can't do it" crew a pass on one thing: They don't always know what type of daycare they'll find /how much it'll cost a year ahead of time, so they don't always know how much they should set aside. For example, our city offers very low cost summer care at the Boys and Girls' Club ... but the number of people who can get in is relatively low (people camp out to get a spot), so no one knows for sure whether they'll qualify for that. Lots of parents "piece together" a summer of care -- two weeks with the grandparents, one week of family vacation, a week of karate camp, then a week of sleep-away camp with scouts, finally a week of day camp at the dance studio, and each parent'll take off some time --
but it's really hard to guess what activities might present themselves next summer, whether your kid'll want to go to expensive theater camp or moderately-priced soccer camp, and what those activities might actually cost next summer. So, yeah, you and I might say, "I'll save $150 for each week of summer, and I'll probably have some leftover', the non-savers often see this as just too many guesses ... so they don't bother.