Why would I get charged for a child's app when that app has been previously purchased by a parent in the same Family?

We have 2 iPads and 2 iphones all part of one family in the latest iOS versions across the board. We had previously purchased a few educatonal iPad apps for the kids, and now that the kids have their own iPad, we went to download some of those same paid apps on the new iPad. Lo and behold, i ended up getting charged for the purchase. so now weve bought these apps twice. Any ideas?