Build a double first. You can carry the youngest in it for a few years.
Build a second double right after. Then build a single.

For speed of construction build the hulls for both doubles from the same plans, but change the cockpit configurations. On at least one of them you want two very open cockpits, or one long, single cockpit. This way you can carry the youngest next to an adult, and you can adjust your seats for better balance if the boat has two paddlers of greatly different weights, as you would probably have with father and youngest child.

The second double and the single can have more tightly fitted cockpits.

With 2 doubles you have one adult in a boat with each child. The parent can paddle and steer (rudder is a must) from the stern when the kid gets tired, and the kid, being in front is always being watched by the parent.

Should one boat capsize the other one (steered by an adult) can approach for rescue.

With a triple you have a longer boat which is more difficult (not impossible, just more challenging) for a single person to handle, or an adult and one kid. You really need two adults in that large of a boat.

Look at possibilities that only a few from the family will want to paddle at any given time:

If both parents are not free, then a triple becomes a liability. (You are either a long way from your youngest, or you'll have an interesting time balancing the boat) Obviously you won't let the 3 year old out alone for a few years, so that child rides in a double with one or two adults.

If one adult is cooking the steaks on shore, the other adult can take either both kids in the double, (9 year old paddles, 3 year old is baggage) or the youngest in the double while the 9 year old paddles the short single.

If the youngest is with friends, the 9 year old and one adult can go in a double, while the other adult goes in either the shorter boat, or solos in the other double.

In future years the doubles become even more practical than a triple. In just 5 years your 14 and 8 year old will be a paddling team that will challenge their parents. With two identical hulls it would be an even race (yeah, right! ) by that time, and for a decade after, the matchups may be one parent in a single, the kids in a double, and one parent soloing the other double.

Beyond that you'll need a new single for the spouse, as your kids will be takeing their dates out in the doubles.

hope this helps

PGJ