You have my sympathies. Have been through similar with my whippet bitch.
She has always been fine with everyone. Doesn't particularly like strange men in the house (a couple in particular she really hated). Once she gets to know someone she remembers they are OK and will go for a fuss if they come round.
A couple of years ago I was expecting a delivery and of course it was scheduled to come school run time. My dsis nipped around to wait in while I ran to get dd. Gone about 20 minutes. When I came home dd was outside with 2.6 yr old nephew. When I asked why she said he had 'pulled the dogs leg and really hurt her'.
Was annoyed at dsis. Checked dog over and she was fine, dsis went home.
A couple of weeks later me and dp went away for the night. It was the first night we had been out since ds had been born 18 months before. My mam had the dcs.
While I was out dsis came around with dnephew. He apparently went to 'hug' the dog and she went for him. Grazed his face and was apparently 'really, really nasty'.
As you can imagine I was devastated. Really pissed off with dsis. Worried about dog around my own 18 mth old ds and dd and her friends.
I bought a crate and crate trained her. Stopped dsis coming around unannounced so I could ensure dog was away before they came in.
When they do visit now dog is visibly different to nephew to what she is other dcs. She is.always crated when I have children younger than 8 in the house. Never bothers her. She just has a snooze. When nephew is here she is stressed, anxious and shows lots of fear behaviour.
She obviously remembers he hurt all that time ago. It's not toddlers or preschoolers or boys she is frightened of. It's that particular boy.
What I suspect might have happened with your dog is that one of your dcs friends might have accidentally hurt him. Or something else hurt him while strange dcs were there. So now he thinks strange dcs in house = ouch.
The problem with whippets is as well as being very tactile and strokable they are also a bit delicate compared to say a spaniel of the.same size.
Crating is the only way to keep everyone safe until you know what is going on and get some help
And sadly unless you can 100% trust your dcs to crate him before anyone comes in then you can't let them have friends in afyer school.