Disagreement about where to spend money

My ex-husband and I have an 8 month old and a 2.5 year old. Last year during the summer he talked me into taking the toddler out of float lessons and promised he would teach her to float and swim. I told him this week that he hasn't been doing it, they just play (and he introduced her to floaties which I didn't agree to) in the water, since he's not teaching her life saving skills she needs to go back to lessons.

he was paying half of float lessons originally, now he refuses to pay half and says it's unnecessary. it's only because he can't afford it he says. I see it as very necessary for safety and accidents do happen when you least expect them. and he went out of his way to mess up the progress she had going, she floated 3 times with an instructor before he got involved. now he's offering to take her to the lesson, which be never did before, but won't pay for it.

on top of that he never takes the 8 month old solo, but he insists on having one on one with the toddler... the dentist appointments I schedule on his day one in one with her (and float lessons would be scheduled that day as well) "interrupt his daddy daughter day". ugh.

I think the 8 month old needs float lessons soon as well

how do you deal with disagreements on where money is spent for the children? do you have any examples of compromises you've made in the past?

we haven't even been divorced a year and this is the first time this has happened and I want to make sure it doesn't happen again later.

I'm the custodial parent and we have 50/50 custody. he requires the toddler two nights a week.