Shipping the bikes there and back will usually be cheaper. If you can't find a Mustachian that will lend out maybe you can find a generous cyclist through a bike forum that will let you borrow bikes or rent for cheap.
Near Ft. Lauderdale, FL.
Spinlister had 2 adult bikes in the general area, go would be about 100$ a week to rent.
It's crazy to think about buying 2 kids bikes from Walmart for the week, when that would cost more than their actual gifts. But I also know that the kids would get way more pleasure from a week of bike riding, than a week of putt putt golf or movies at the theater, or really anything else I can think of. Even if we only buy kids bikes, the adults can walk. That way we can get 3-4 miles. With the kids walking, they make it about 2 miles at a slow pace, whining. Looks like the cheapest choices from Walmart are 49$ for a 20" and 35$ for a 16". That's assuming we can find those models in store, they are sold out online.
If there is anyone local who has kids bikes, I would rather pay you than pay walmart! I'll call local bike shops too, but I'm going to wait until closer to the trip.