Car seat issue

So I'm going on the dream at the end of August out of Fort Lauderdale. We will have a 6 year, 4 year, and 11 month old at the time.

Obviously, we need a car seat for the 11 month old.

What's the cheapest solution?

The hotel is only 2 miles from the airport. And then the port is only .5 miles from that.

Curious what other people have done