my kids are 9 years old in the game lol, when can they start working on the island?

they can start working at 14

You're right Gemelle, they are pretty much useless until 14 when they start earning thier keep. They can start breeding at 18.

It would probably be more realistic for kids to make zero contribution up to around age 6 and then make fractional improvements until around 15 when they're pulling full weight, depending on the task. Learning at an adult's side could increase their learning curve for various tasks. Naturally, "full weight" would depend on their natural strength levels. A 15 year old would be weaker than a full-grown 22 year old but stronger than an 80 year old farmer.