I pay for a subscription to a math website called IXL.com for my children to do math during the summer. I like this particular website because it is progressive and tracks everything they do. I get email reports of what they have mastered, how much time they have spent, etc. With so much going on, following up on each child can be a challenge. I take at look at their summer scholar journals and get this. The best thing, though, is that they like to work on this website each day. 15 minutes is a short time (they often go longer because they are enjoying it), and it keeps their math knowledge base maintained and growing.
Another great website is XtraMath! Anna used this is third grade and it was great! It is FREE. I haven’t looked into it recently. I need to do that to save money! I’m embarrassed to say when I couldn’t remember it after googling/looking for it, I defaulted to IXL because we had used it in previous years and liked it. I may try the mobile app and see how that goes, just for waiting moments at the dentist or wherever when my children are waiting without a book on hand.