Re: discipline....sneaky kids...help!!!
I would def make them sit with you while you pump. On the floor, criss-cross-apple-sauce, rdg a book, or sitting silently. And I would explain that they lost the privilege to play w/play dough, or watch a movie, or color while I was pumping BECAUSE of their bad behavior. I would make sure they were bored, and wouldn't allow any playing or fun activities while they are sitting with you.
This is a natural consequence of losing someone's trust, you lose privileges. The 8 and 4 yr old KNOW better than to do that stuff, and they are even old enough to stop the 2 yr old from getting into stuff, or at the VERY LEAST, telling you when the 2 y/o is up to something.
When my DD tells us a lie, she has to sit right next to us for a whole day (or two, depending) - as in, I go potty, she stands in the corner and waits, I wash dishes, she sits at my feet, I do laundry, she sits and watches. I know she is bored and I know she can't stand it, but when you break my trust, then I can't trust you at all, even to be alone for 2 seconds. That's what happens.
And in a way, IMO, this is deceptive b/c they are waiting until you are gone to misbehave. Not the EXACT same, obviously, but I think it is similar enough to treat it with the same method.