Consider buying tandem (double) kayak - especially for long trips.

They are faster and you can talk, not shout, haul a lot of gear and
generally have less hassle than w/two kayaks.

Later you can pick up used or new single, when you now what to look
for.
Other way around does not always work.

You can alternate double and single w/your kids, instead having to
deal w/three kayaks.