Jazzy does not like it when kids come to our house. {She's fine with them elsewhere}. I don't let her greet people at the door anyway, for fear of escape – I have had adults let her out -- and when kids come over I crate her for the duration of the visit, except for her potty breaks or if the kids are outside. She gets a treat and a nice long nap, and they play w/out fear of being bitten.
She's usually okay if she is outside w/them, but I don't take chances. I keep her in when visiting kids are out, and put her out to potty when they come in to play.
Kids she knows well are not a problem.
As far as an aging dog beginning to behave this way, maybe he's just older and tired and can't deal with them -- think of an older person when unpredictable kids are around; they are more tense, etc.
If he proves to be healthy, I would just keep him away from kids when they come over.
Hell's bells -- sometimes when kids come over, I don't feel like I am up to dealing with them well either. I don't bite them, but sometimes it can be tempting. LOL And I'll send them to another room to play so I can relax -- kind of like crating myself, I guess.