Tier 2 dependents

Hello,
Can you please help me with these questions?
I am on Tier 2 general visa for 5 years that expires 14 Feb 2024. Both my wife and first child got thier dependent visas and joined me after 4 months after I entered the UK.
1. Will my wife and child have to wait for 5 years from the date they entered the UK to apply for IRL?
2. My second child was born in the UK in 2019. but I want to apply for his dependent visa as we might travel abroad. but I am not sure which fees to pay; for 3 years or more than 3 years?
It's a bit confusing to pay for a visa for a child before he was born!
3. If my wife changes visas (to Tier 5), would my sons lose their choice to apply for ILR if/when I apply for one in 2024?
Many thanks for your help.

1. It's 5 years counting form initial visa issue date.

2. It based on your cos/ visa duration.

3. Yes,they would. Why would she want to change to tier 5??

Note a UK born child does NOT need ilr if you plan on registering the child as British.