A couple things to consider. Spiders and critters that live in crawlspaces are not the greatest thing for kids to be around. Black widows and brown recluse spiders live in crawlspaces. The second thing to consider is when you enclose the area you will be upsetting the cross ventilation of the foundation area. I wouldn't worry too much about the kids bumping their heads, they seem to dojust fine at those play structures in McDonalds without splitting their heads open.