I'm assuming these mares have been together before they foaled??? Mares are notorious for this. One reason why I put Cool up when I put Red up to foal, because with Cool being the alpha mare, I knew she'd try and steal Red's baby - sure 'nuf Cool dropped her milk when she'd hear the baby - she would've for sure stolen him.
If you have a ton of land where mares can foal outside, that's much better; however, there's always something that could happen. If a mare stays off by herself away from the herd with her baby, she's usually okay; however, if she's not the alpha, you may have some problems.
Once my mares have their babies, I have them in separate pens but beside each other. That way everybody knows what's up, and usually in about a week, I'll let them back out. Now, when you start getting a bunch of mares, this can get kind of weird... better to pair them up rather than sticking one out at a time. When you feed, you have them WAY far away from each other... that's usually when all the broohah starts is when they're up by the barn waiting to be fed or during feeding. Alpha mare gets fed first, then the next guy, then the next guy, etc.
As far as babies flipping under fences ... some I can understand how they do it, others, I haven't a clue, but they do it. If there's 3 inch hole anywhere, they'll find it. And any time you put mares and babies beside each other, make gosh-danged sure, that no little legs can get in the opposite pen and get stepped on - that's my worst fear!
I know I have a friend that raises minis and she has a bugger with that, so she started cutting up plyboard and putting it all along the bottom, about a foot up - makes sure shavings, dirt or whatever is sort of hilled up so there can't be any openings whatsoever. This system works great if you have to use panels.
And, no, I haven't seen a filly quite act like that, unless she was just romping and playing glad to be out .... they'll run like a turkey and then all of a sudden start screaming for their mom - act like they're confused. Mom screams back and all is well with the world.