Wow, I thought stupid parents only existed on this side of the pond.
Sounds like you and your pups were way over your thresholds. Stress added to stress can sometimes look like creative math where 2+2=8. So in the case of Foster, he was over (or very close to it) threshold after dealing with the staffy. When he encountered his "friend" his "friend" may have done something that, on a normal day, would have been a mild annoyance. But, the mild annoyance coming on the heels of the staffy incident was too much for Foster and he lashed out. The straw the broke the camel's back so to speak.
And that applies to you in dealing with the loose child. Do you think you woul have lost your cool if you'd had a wonderful day at the park? It's like some days someone will cut you off in traffic and you'll grumble, other days (really crappy days) something like that will happen and you'll start yelling words that would make a salior blush. (Okay, maybe not you, if it helps, you can substitue me instead.)
The reason I'm telling you all this, is that it can take time for those stress juices/pheremones to return to normal levels. So be on the look out tomorrow, perhaps the next day, for stressors in Foster's environment. If you're headed to the park and the loose staffy (or annoying child) is there, you'd probably do best to just take a nice walk somewhere else.
And I'm sorry your nice day at the park was ruined. Hopefully things will look up tomorrow (which is probably already there as I type this right now)