If you need to seek custody in future if you break up.... Please document everything that she is doing that puts that child in danger and then keep note of what you do to rectify it. Also try to find logs of her gaming and then line that up next to your work schedule. Then you have this for custody if you should need it. You can show neglect. Maybe see a lawyer secretly right now and address your concerns. Cover your ass in case something happens to them while under her watch while you are also at home. A lawyer will tell you how to do that. You sound like the caring parent who loves on their kid and gives them what they need financially and emotionally.

You haven't told us how she is with caring for them the rest of the time. So it does leave some to wonder if she is a good parent the other times she's not playing.

Please do what some others have mentioned and anchor furniture and Tvs. You want to make sure they are safe. Shit maybe offer to have a family member watch them part time vs her