Even cellulose (basically paper fibers) is bad to breathe in. Mineral wool (which is what it looks like) isn’t good for the lungs. A little here and there isn’t that big of a deal, but I wouldn’t make a habit of breathing it in. Especially for kids with developing lungs.

Pretty much all insulation products are harmful if inhaled, and every time a ball hits that stuff, what you are not seeing is small particulate being broken loose and sent airborne… to drift down over your kiddies heads. Probably not life threatening in limited exposure, but definitely not beneficial!

If it were me, I’d screw up the cheapest sheet good I could lay my hands on, or as others have suggested, staple up some 6mm poly and tape the seams with tyvek tape (the red plastic-y stuff). 6mm is pretty robust, so it should stand up to some abuse, but if you go too thin somebody is bound to poke holes in it eventually.