There is no doubt the Seahawks QB is ALL IN for the stepdad life after seeing this video -- he proudly walked around in a Mickey Mouse "Fantasia" hat ... and you gotta see his reaction to a roller coaster in California Adventure park.
The newly engaged couple and the little one look like the perfect family -- which they'd kinda have to be to brave Disneyland crowds on a holiday weekend!
Another explanation for Russell's screams? That whole abstinence thing. Let it out, bro ... let it out.