Thanks for all the responses while I was on vacation. Turns out that beach was not driveable (out of season), so we didn't get to ride/drive on the beach. I was expecting Long Beach, WA to be like Ocean Shores, WA where I drove my car along the beach a few years ago and the sand was hard packed and definitely driveable. But this the wrong season. Driving on the beach in Long Beach, WA is only allowed between Sept and Apr. Anyway, there was a nice paved bike path set back from the beach that we rode on instead.

So no sand/saltwater exposure to either bikes and the training wheels stayed on. My son got to experience short/steep rolling hills instead.

Driving at Long Beach, WA for those interested: http://funbeach.com/activities/beach-driving/