To save money book 2 Main cabin not E class. Get the immediate upgrade to Comfort+ if you are PM or DM. Then book one of the kids by themselves and pay for Comfort+ and select a seat beside you. Call DL and ask them to link the 2 PNRs. The kid with the separate reservation won’t be able to do OLCI. If OLCI is important to you then you will need to pay for 3 Comfort+ seats on the same PNR.