Adopted springer spaniel advice needed

[MagpieCursedTea]

I was hoping the wealth of dog knowledge in the doghouse might be able to help me. Well, it's actually my parents I'm asking for, they're seeking the advice of the shelter tomorrow but I thought we might be able to get some advice here too?
They currently have a 8 year old male springer spaniel on trial. He's not been neutered and has come straight from his previous family rather than being in a shelter (but the adoption is going through the shelter). He is being rehomed because the previous family have a 2 year old grandson who is allergic and they were looking after him more often so the dog was being shut away and they thought this was unfair on him. We were told the dog was never aggressive though he does growl, it's in a slightly argumentative way but with the tail wagging if that makes sense?
Anyway, my parents look after my 2 year old DS whilst I'm at work so were choosing their new dog very carefully and decided on this one due to him being used to children (this was obviously discussed with the shelter as well).
My parents have handled introductions slowly and carefully and DS and the dog have been getting on very well.
The problem has been that on a couple of occasions when DS has been upset and my parents have gone to pick him up, the dog has tried to stop them (a bit like he's trying to protect him?) mostly by growling but today actually snapped at Dad and caught his hand. The dog was immediately sheepish. He's a lovely natured dog and is generally very well behaved and hasn't shown any aggression towards DS, he's very gentle and cautious with him. However my parents are understandably concerned. They're experienced dog owners but we're not quite sure how to handle this. As I said, they're going to speak to the shelter and my parents are wanting to resolve the issue if possible but don't want to formally adopt the dog if there are going to be any risks relating to DS, both for his safety and the dogs welfare.
Sorry for the length but I'm trying not to leave anything out. As I said, he's a very lovely dog (I'm especially quite taken with him!), has anyone had a similar situation that has been resolved? We don't want to give up on him if there's a way to deal with this but equally don't want the dog to be in the wrong home for him.

I suspect the previous owners haven't been entirely honest. The dog may have been shut away eventually but I wonder if at first they have let the toddler and the dog be in too much close contact and the dog now associates an upset child with bad consequences for itself.

I think this dog would be happier away from young children tbh. My grandparents had a number of springers and only one tended to snap. they are beautiful and amazing dogs I think - but definitely not for everybody.

Thanks for your reply. A friend of mine also said she doesn't think the people were entirely honest. They knew about DS though and we met them with DS there so would hope they'd say if there had been an issue

They want a nice home for their dog though - and if we're right they have screwed up. You can't trust what they say. Trust what you see - which is that a crying child really upskittles an otherwise serene dog.