The main problem with having it in the back is that once the tank is full it'll be hard to access it. With a 4 inch wide UPS, you'll probably need the stand to be further from the back wall. Knowing your big space there, I think a better idea would be to setup a childproof system on the side. There are storage stools and benches that look like furniture (and your kids can look into the tank on it) that people use to cosmetically hold ATO and water containers for water changes via dosing pumps. You could easily find one to fit your UPS and lock it down so you have access to it, keep your kids away from it and don't have to put it in an inaccessible location behind your tank.