iPad Mini repeatedly querying apple-dns.net

I have a filter on my home Wi-fi that lets me control how long my children spend on line and what sites they visit. Last weekend one of my children's time limit got used up when she was gone all day. Investigating I discovered her iPad mini is repeatedly querying apple-dns.net. I've confirmed that the domain name is registered to Apple. Rather than either white list the site in my filter or just block it completely I'd like to know why her mini is doing this. There are 4 iPhones, 3 iPads and a few Macbooks in my house and the Mini is the only one with the problem. As far as I know there were no changes last week when the problem started. Is there a way I can figure out what app is doing this or why it's repeatedly querying Apple's DNS server? I've shut off the iPad a few times but when it's powered back on the problem starts up again.