J.R.
I live in the San Juans! What time of year are you coming up? That will dictate how early you need to be in the ferry line. If it is in summer, bring lots of books, snacks and playing cards to help make the wait fun.
The easiest island to access is Lopez (first stop) That is where I live. Our island is rural, beautiful and peaceful. There isn't alot of town, but the vibe is great!
Orcas island is big, great hiking in Moran State Park. San Juan Island is the county seat and has the town of Friday Harbor as a feature. It is easiest to go to without a car.
All the islands have great kayaking, bike riding, whale watching, beach combing, relaxing.... If you are coming for several days, you can stay on one island and "island hop" for day trips.
Please message me with more questions! I love to share our islands :) It really is an incredible place. You and your kids would love it here!