How Do I Nicely Tell The Exwife I Don’t Want More Financial Burden?

I (30f) have two step daughters (11f and 9f). My husband (36m) and I are on good terms with the ex wife (32f) and we have an arrangement where we have 50/50 custody. That means we split all expenses in half I.e. daycare, sports fees and equipment, doctor visits. We get along except when it comes to money, of course.

You see, my husband and I make more than her and her husband and we've always been more financially savvy. They tend to jump from job to job and rack up tons of debt. As a result of this and the fact that most institutions aren't set up to split charges, we usually cover the expenses and she pays us back. She does pay us back... eventually. It used to be worse when they kids were younger because they'd have to go to full time daycare and that would be charged every week. When we had her pay for daycare and we'd pay her back, we'd get charged late fees because she wouldn't have the money and it'd bounce. So we started paying for it (again, most schools aren't set up to handle split households and bill each portion separately), but she'd take forever to pay us back. At the worst, she owed close to $1000 in expenses to pay back as it had been months of no payments. Not even $100 here and there. To further illustrate their bad money choices, they got a doodle mix at that time. I'm not sure if the dog was free or they bought it but either way, I feel like we should have been paid back before bringing another mouth to feed into the mix (and shots, neutering, vet bills). So things like this have been pretty stressful on the relationship we have with her. Not on mine and my husband's because he's sick of it too. We feel like we're being used as a free loan because what are we going to do? Not pay for daycare or a Dr bill and have it go to collections and ruin our credit? No. She's not doing it maliciously, it's honestly because they aren't good with money and we're the safety net when it comes to the shared Kids.

So my problem here is that every year for the past 2 years she's been asking to get the older kid a cell phone. We said no because we don't feel like a 10 yr old doesn't need a phone and if it were up to us she could have one at 14. Plus, she's not in outside of school activities like 9 is so it's not like she needs to contact us outside the house very often. She's got a tablet with Facebook messenger so she can still contact us if she has wifi. We compromised and agreed on 12. Well last year she asked again about getting 11 a phone for emergencies when she's home babysitting and if we could put it on our plan because their plan is full of lines. We said no because we still aren't comfortable with her having a phone but I can't tell them what to do at their house so if they want to pay for a phone for emergencies at her house so be it.

Well, she turns 12 soon and I know the conversation is going to come up about getting 11 a phone. We'll stand by our agreement that 12 is ok and we'll help pay our half. But I'm not ok with it being on our plan because of her history of not paying us back. I'm not excited to sign up for a monthly phone bill when she already can't consistently pay us back. Currently it's been over 3 months since she's had a zero balance with us.

How do I nicely tell her I don't want to sign up for a monthly expense when she has a history of racking up debt with us?