My kids are nearly 5 & 3. We have a "standard" visitation agreement - every Tues afternoon & EOW, I believe.
However, X doesn't stick to it. On average, he probably sees the kids slightly less than once a week, for a few hours. He rarely schedules a visit of his own volition - either his parents push for it, when they schedule a visit, or my older child calls him & asks him to come & visit. He only takes the older one for overnights, and has only done so once in the past several months.
It's really crummy for the kids. He does approximately zero "parenting." They're social visits in every sense. Not having any routine with him stinks for the kids, and for me frankly. I WISH I could plan regular time when he would have the kids.
In terms of "holding him" to the formal visitation schedule - I could *limit* him to those times, but could not *make* him take the kids at those times.
ETA: your other questions -
They would like to see him more, not less.
What is reasonable, IMO, is having a consistent routine where both parents are actually parenting, hopefully with a united front.
I'm very flexible. I have told him that he can't expect that the kids will be available on less than 48 hours notice. That said, I will pretty much always agree to them spending time with him unless it interferes with plans already in place.