They could climb on him, roll on the floor with him, pull his tail etc. Well, guess what! Like you, your dog does not like to be crowded or crawled on and wants space. The difference is you and I can say nicely say to our children...."I need some space" or "not right now." (Notice this dogs closed mouth, whites of the eyes, ears back....all subtle signs.)
Dogs (if you haven't noticed) can't speak up in a way that is clear to us. They will indicate...Hey kid, move over by licking their lips, turning away, whale eye etc. But often these communications, that other dogs understand, go unnoticed by we humans. Then once these signals don't work the dog will move on to growling or snapping. They too get fed up. (Again, this photo shows a dog giving subtle signs...ears back, lowering herself, whites of eyes, and closed mouth)
Think of it this way. Would you say this about a guest that comes into your home? "She is so great with our kids...she allows them to hit her, pull her hair, jump on her back and never complains." I don't think you would. I also believe you would be horrified if your child behaved this way towards another person. Why is it we say our dogs are great with the kids and then list all the things they "put up with?" Why? What are we teaching our children with this mindset? This is a HUGE pet peeve of mine as it is a very inappropriate expectation for our family dogs.
As parents it is OUR responsiblity to protect our dogs and children as they learn how to respectfully interact with one another. That means, gentle touch, supervision and allowing your dog some time to Chill out away from your children. (Notice this dog's tail tucked, lowering of the body and curving away from the children)
I encourage you to think about what makes your dog a "good" dog in your eyes and think about what you are expecting him to "put up with."