Open pool or No,no

I'd want to avoid wet kids running through a reception, so I'd keep it closed too. If you wanted to open it, I'd try to get some childcare to watch over them and make sure they're dry and changed before rejoining the reception.

I think this would depend on the time of day the wedding is. If this is a middle of the afternoon / early part of the day wedding, then I think it'd be nice to have the pool open for the kids. Yes, sometimes kids go "unsupervised" at weddings, but most weddings don't have a pool. I know that I might let my 5 year old dance with the other kids on the dance floor without watching her like a hawk, but if there is a pool around I will have my eyes on her at all times. Plus you said there is a life guard.

Unless someone is specifically there to watch the children (other than the lifeguard) I think it would be difficult for parents to relax and enjoy your reception with their kids running around in and out of the pool. It is a very nice thought but I would recommend considering the logistics before opening it up.

I would leave it closed, unless you would have an adult in the pool area that would be willing to watch the kids. They can get pretty rowdy and loud so unless someone would be there to supervise besides the lifeguard it might get a bit crazy.

Not kid related, but... if you have rowdy guests (like a groomsman that likes to party hard after he's had a few drinks), leave it closed. The last thing you want when done up all fancy in your wedding dress with hair and makeup is to get thrown in a pool by a goofball guest.

I agree with PPs. I wouldn't want a bunch of wet kids running around my wedding