I have a family of four, Mom Dad, 4yr old and 7yr old. I am debating between the following:
1. (1) Tandem Island
2. (2) Oasis
Which would you choose and why?
My personal choice would be the Oases. Reasons -- easier to handle, more manuverable, more loading and transport options. BUT
--- I took my 7 year old nephew out one day in my Oasis. He thought it was OK, lost interest in pedaling in about 5 minutes and got somewhat bored after about 15 minutes. A few days later I took him out on my Adventure Island (put him in the cargo well) and we went sailing and did a little informal racing with another AI. He was thrilled, found the whole experience exciting and hated to leave. He got to try his hand at sailing and was fascinated.
So for your family I think the TI would be a better option. No one has trouble keeping up. Your kids can pedal, paddle, sail, fish, use it as a swimming platform or just hang out on the tramps. You can even tow them around on their own raft. The boat has plenty of capacity and versatility. It's a blast to sail solo, perfectly suited for two yet fully capable of handling 4 adults. It even does a respectable job as a solo (non-sailing) kayak without the amas. It's only disadvantages for some would be its weight, transport handling and storage limitations. As Bob mentioned, it also makes a good "mother ship" if you want to add another kayak or two down the line.
What ever you decide, it will be a great family activity. IMO, it's more important to get started now than try to make the perfect decision later!