2gypsies, I agree, renting would be idea. Unfortunately, only the state park campgrounds have kayak rentals (listen up, campground owners...you're missing an income opportunity!!) and they will not allow children under 8 for safety/liability reasons.
When we last camped in Chincoteague, my daughter and I drove to a nearby harbor and went out in a rented tandem kayak, to the tune of $69 for 3 hours. Ouch! It was fun to pull up on the sandbars and see the ponies and birds up close, but we could have launched from the campground if we had our own kayak, and saved time and money.
Jebby14, I never thought of tying all the way down to the bumpers! Alas, I think it is a moot point because my husband has said no way to putting anything on the roof, with foam blocks or not.
I'm researching reviews and options now. It seems that some inflatable kayaks are OK if they have a skeg, or if you add one in order to improve tracking. Certain brands seem to have better reviews than others for maneuverability and durability. I think my next most viable option is a smaller rowboat style inflatable, but I need to consider how much effort it is going to take to paddle. I don't want to invest in a trolling motor just yet. Initially I said we may use it 5-10 times, but that is a generous estimate. Honestly, 3-5 times a year is probably more accurate. And, with a 5 year old and 9 year old, attention spans are small. They will be enthralled by watching fish and turtles for about 30-45 minutes. I'm not sure that I need a lot more than a glorified pool raft and some paddles!