Tier 1 (Exceptional Talent) visa : Passport Validity for a child

I need a help. I am successful Tier 1 Exceptional Talent applicant.
With this visa I came to UK in the month on November 2017 and have been doing great.

Now I am planning to apply for a visa for my 3 year old child and my wife. For my wife I am applying for five years visa as she has a passport that is valid for next 8 years. My query is regarding my child.

My visa is valid for another five years (Mar 2023). Can I apply for a visa for my child for five years. Problem is that he has valid passport till July 2021 which is three years from now.

Please suggest an option:

1) Visa for 3 years
2) Visa for five years

~Regards

I am applying for three years as with this time I can apply for ILR. And then dependents can also follow same route.

My understanding is that dependents apply for and get the same visa validity as the main visa holder.

Did you not get a BRP card to collect in the UK once you arrived??

PBS Dependent visa holders require 5 years residence to qualify for ILR as far as I understand the immigration rules.