This happens with friends x 2 (when it starts off with kids being friends and parents being friends).
Either the kids start hating each other (too much time together with someone you only kinda like blossoms into out and out playmates -> frenemies -> enemies.
((Either you cut out the kids and spend grownup time, or the adults end up losing their friends because it's too hard not to get emotionally involved in kid fights))
OR
The kids are great and the adults start hating each other.
((And the kids lose their friends, because the adults can't stand each other. No fix there, unless they go to school together))
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These girls spend a LOT of time together (every morning and 2 days a week, in addition to whatever time you spend together as families). Even if they were besties that would start to put a strain on the relationship. And they're NOT besties. It's starting, instead, to put a strain on your relationship with your friends.
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Nope. I wouldn't say anything to the mom, because this is a simple case of the girls not being friends, and still spending a lot of time together. Natural frictions are developing in regards to their different personalities. TOTALLY to be expected, and I'm sure there's "blame" on both sides / aka no blame really, just 2 very different girls whose parents keep putting them together.