A question about saving for kids when parents aren't together

[Pippipumpkin]

Hi, I would really like to have some opinions on the following.
Me and ex have two DC. When we were together we opened savings accounts for them and paid a small amount in regularly. When we split I have kept control of the savings accounts (because I set them up etc.) and have continued to pay a small amount in each month (which has reduced due to my single wage).
Me and ex co parent amicably, and I have the DC 60% of the time. He pays maintenance to me based on the CMS calculator.
Following a conversation with someone at work (who was in a similar situation) I am wondering whether I would be unreasonable to ask ex if he would pay into the savings account too (I am 99% sure he won't have set up a separate account for them). I think it would seem like a nice show of unity if we save together for them, but likewise my practical future planning head thinks we both need to be saving for them (I know I can't make him, so if he says no that will be the end of the discussion).
Anyone in a similar situation?

What’s the relationship like? What are his feelings about paying maintenance? Can he afford it? What do you think he is going to say? If it’s a good chance it’s a no then don’t.

Relationship is fine as these things go, no pettiness, can be flexible to each other.
He never talks about maintenance, just pays it.
I think he is comfortable enough. He lives with someone else in a situation where I dont think there is much mortgage and he earns a similar amount to me (which isn't loads, but is ok for the area).

I always second guess myself when asking stuff like this though.

I didn't think maintenance was intended for savings which is why I am thinking about it.
If we were still together we would probably still be saving more for them (albeit driven by me)

You can ask but I doubt your ex will save in an account controlled by you. I would suggest a Junior ISA on each child's name so neither of you can withdraw money. I think this would be a more palatable option for him.