Lots of good points made here in the responses. I agree that the kids will see what is meaningful in the long run. Good thing they live with you so they can see what is REALLY important in life.

Gotta ask why you let him do this to you. Do you have to talk to him about what they are doing? All you really need to know is when and where to drop your child off. We have a set schedule and our son is old enough to handle plans on his own. He does all the talking to his father. If your child wants to tell you what they are doing or did is one thing. I don't ask. Tell the Flea that you don't need to know what they are doing or just don't give him the opportunity to tell you.
If you have to see your ex while dropping off or picking up your child/children make it as brief as possible. Take away his taunt you if that is what he is doing. If it is just an explanation (which I doubt since he doesn't give them unless BF is going to be there)we have to accept that we don't need them. What our ex's do with the kids during visitation is none of our business just as what we do with the kids is none of theirs. As long as the kids are not being harmed in anyway.
Hope things get better soon.
You will be much happier.

Good points. Getting the right things packed can be challenging if we don't know. I tell my stbx if they need anything different than normal for the weekend, e-mail it to me. I know he's had to buy new swim suits before.