I lost my son when he was 18 months
We were at home and I'd gone to the loo. I couldn't force him to stay in the bathroom with me. On this occasion he'd succeeded at climbing over a baby gate (he'd never even attempted it before to my knowledge, but succeeded on his first try!), made his way down a flight of stairs to the front door, unlocked it (key was in the lock), walked down our street, crossed a road... all to get to our local playground, while I spent 10 mins in the loo thinking he was still in the living room watching Thomas & Friends!
I got back from the loo and saw he wasn't in the living room. Searched for him and found him nowhere. Then the frantic panic set in when I realised he wasn't in the house. Then I saw the front door open and ran outside barefoot screaming out his name in tears, utterly beside myself. It was the longest few seconds of my life. I've never felt fear quite like it.
Then I spotted him at the end of our street, in the distance, now attempting to scale the playground gate!
I spent the rest of the day in tears So angry with myself, feeling so unbelievably guilty and ashamed. From that day on all keys in outer doors were removed and placed out of reach after being locked.
But several years on, looking back, I realise sometimes these things happen. It doesn't mean it's bad parenting. It's IMPOSSIBLE to constantly have eyes on your child 24/7. And kids aren't daft, they know when to take their opportunities! (Like when mums constipation is causing her to spend longer than usual amounts of time on the bog!)
Your DS will be perfectly fine at the party.