I've never been keen on turning a foal out with a slip on. The mare has a 'safety' headcollar. Is there such a thing for a foal? I've searched on line and the foal slips seem to be designed to be robust !
I've always used leather foal slips and even the shettie foals can break these if they try! If for any reason the leather does not break, the stitching will. Hope that helps.
You can get safety headcollars for foals; I can't remember where I got mine from as it is a while ago but I bought them on line. I bought 3 and still have 2 unused if you really need one. I have the cheque stub but didn't write in who it was to!!! You can pm me if you wish.