It is absolutely normal, but should be possible to train him out of. It will be harder with the two-year-old than the five year old, after all, the 2-year-old probably shrieks excitedly, which is just like being a giant interactive sqeaky toy!

Always have plenty of chew toys around, and hand, so if the pup starts mouthing you or anyone, that person can firmly say no, and give him and okay-to-chew substitute.