A question for you. Why can't the kids "control" the SD PVR themselves at the moment?
The only option open to you is to convert to the 3 decoder setup as Optimist has referred you to. You could then get 2 x HD decoders for granny flat and the kids room, as the minimum
Easyview, UEC 4T HD PVR, SD PVR, XV
Spare decoders: SD PVR(2), PACE HD PVR 4T, DSD 660, 1110, 1131, Explora
2 unmentionable FTA decoders
Win 10 Pro (64-bit) build 1607, version 14393.351
1.2m antenna, 8-way universal LNB, 2x6 MS, FSM permanently connected.