This is a very common issue. Some dogs - and often dogs with more "drive" - enjoy holding something in their mouths when they get overly excited. What breed of dog do you have?
One of my shelties does this on a regular basis, and I make sure he has plenty of "his" toys at hand to put into his mouth during times when excitement will get the best of him. You can teach your dog which toys are acceptable for this behavior (ie his toys) and which are not (ie your child's toys). You can also stop the behavior altogether, although I find it's a very helpful behavior otherwise.
As for the reason as to why, we have intentionally bred this into our dogs. As retrieving objects is a very helpful skill for a working dog to have, some of our dogs' ancestors were carefully bred for their ability and enjoyment to retrieve. Even today, obviously, we choose breeding stock in part for retrieving abilities. In my particular breed and from the line my dog comes from, toy drive is greatly treasured. So a dog with the desire to play with toys and hold them in their mouths would be chosen for breeding stock over other candidates, all things being equal. Thus, this is an inherited trait which you can train and mold to even help you. One of my dog's loves to retrieve my TV remote for instance, which has been a huge help. :)