User blog comment:B214/High School DxD EX/@comment-4547267-20150828180103/@comment-74.197.121.121-20150828182522

The EX Future is the anime original alternative timeline where Loki did his Loki things instead of giving up after Issei's group and Vali's group thoroughly beat his ass.

EX is basically a weird but canon story that sets about fixing BorN and setting things back to the way it should be (basically, resetting so that events follow the LN proper). Which is why it's not necessary to read, because it basically is intended to write away BorN. And if you write out BorN, you pretty much are back on track with the LN (starting from V5).

As far as the children go though, they are confirmed canon decendants of Issei in both the main story and the alternate future. The only difference is that the split happens due to Loki and some other evil gods screwing things over, so the children are working to set it back to the proper future.