Designate Oldest to Youngest Child < 18 years

We've requirement where we need to only allow the employee to cover their children from oldest to youngest child and they can cover upto a maximum of 3 children.

Example:

Employee has 5 children:

A (16 years)

B (15 years)

C (17 years)

D (6 years)

E (4 years)

Employee should only be allowed to cover - A, B, C

Any suggestions would be helpful.

Thanks!

Regards,

Sambhav Gupta