Agree with everyone, including the ex maybe being too connected still. When you said you don't know if the ex will be around - is this because you are really cool with being friends now and he hangs out at your place sometimes, or because he might randomly drop by to pick up the kids? Either way, myself, I wouldn't be worried about my ex knowing all the details of a new relationship - really not his business. Am wondering about the tension you mentioned - is it from your new bf, or your ex? If it's from the new bf, maybe he's picking up on some unfinished emotional business between you and your ex?

BG, you need to set the rules about what your ex does or doesn't do while the BF is there as much as you can. If it's his time to have the kids, then perhaps that would be a good opportunity to do the handoff in a neutral location...coffee shop or some other, where there's no lingering aura of "his" place in your life.

You say you're worried about them meeting...but why would they if you and your ex only deal with each other over the kids? I don't know the size town you live in, so I don't know if it's the kind of place where you might run into ex in the grocery store or at the movies. Other than those coincidences, you should control the meeting. Ex shouldn't just be dropping by...that's going to make any man crazy (and probably for good reason.)

Also, if you and ex are on reasonable terms, I'd have a chat with him beforehand. I can understand you not wanting him in your business, but you've already spent a summer with BF...with the kids, right? So your ex has to know it's more than casual. If not, why not? If my ex had my kids off visiting someone, I'd want to know about it and I'd want to know what it was about.

If you've talked it over with your BF and he's aware of all the circumstances, there's no need to go over and over the possibilities. State what might happen to the BF calmly. State what IS happening to the ex calmly. Let things go from there.

You say you don't want any fighting, but I'm not sure who you're worried about fighting...you and your ex? Just don't do it. This is one time to just walk away from whatever nonsense he might pull, if he's inclined to such things. If you're worried about the two of them fighting, that's a more serious thing. Is your ex violent? Is your BF inclined to violence? Most grown men can manage to get through a meet, no matter how distastful, that they've been forewarned about without coming to blows. It might take one of you biting back a response or two, but that's just part of it.

I hope this helps. I think, unless there's more you're not sharing, that you just need to prepare your ex, prepare your BF, control the circumstances of any predetermined meeting (picking up the kids) and relax and have fun.