Ok that makes more sense. This is how I would handle all that.
1. Her bringing the kids over to get the things they forgot is a good thing because you get to see the kids again and you didn't have to lift a finger. It being unannounced is annoying and rude but brush that off.
2. Don't rush the kids out. If they want to be slow in getting the move on no big deal, its just more time for you to interact with them. If mom wants to rush them then she can do that but you don't have to, you are under no obligation to rush them back to their mother.
3. When the kids say things like that just shrug and smile at them, tell them you love them and to have a great time with mom and then remind little Johnny to not forget to brush his teeth before bed and remind little Sally to not forget about her class project that is due next Tuesday and if she needs any help to give you a call so you can help her with it.
I do agree with Trevor on teaching the kids to be punctual and prepared but at the same time we are talking about kids, things will be forgotten and things they didn't want when they left they suddenly want when they are gone.