Sounds like a huge job, either digging them up or cutting them down, then digging out the roots. Could you maybe just fence around that area until the Sagos are big enough to not be reachable by the dogs? They're really beautiful and easy care plants.
Just thinking, there are a LOT of plants that are toxic to pets, but most people don't worry about it because their pets don't ever touch plants. Seems to me that anything you plant there could be a danger to your dogs until they learn that plants aren't on the menu. Don't you have trouble with this when you take them out to exercise, too?
A friend of mine trained her dog to leave the plants alone in one day - she sat on her patio with the hose sprayer in her hand, and squirted them lightly whenever they sniffed at a plant. Of course she rewarded the dogs with treats when they kept away, too.
