Your dog is behaving naturally-- he's defending "his" territory, which includes the garage and yard. It is not just being aggressive, but the dog also being afraid. Also, if you are hot in there (and cranky?), imagine what he feels. A dog trainer could give you tips on how to reduce any aggression. I'd suggest putting him on a leash whenever you have the garage door open. Give the kids the bestest toy to offer your dog -- or best dog treat to toss his way. Buy the best short control leash you can get (no spiked collars though!), and invite the kids to come on short walks with you and the dog. And you and the kids play together as much as you can. The dog has to see the kids as belonging to him so he protects them, not as intruders to protect you against.