After three years we can apply for their citizenship thought MN1 application. My question is , do they need to do biometrics again?
Biometrics are required for every application you make to UKVI. So yes, it will be required again.
and do we still have to pay the full fee if going from ILR to MN1 ?
Yes, the full fee to register as British, which is currently £1214 per applicant.
I also heard that we can apply after 2 1/2 years and not wait the full 3 years , is this true ?
Not true. For a Section 3(5) registration, which is what you are applying under, you need to have been resident in the UK with the children for the full 3 years AND be within the absence limit. Not sure where you 'heard' about applying after 2.5 years. Always refer to UKVI official guidance etc.
From the official guidance notes, page 10 and 11.
https://assets.publishing.service.gov.u ... t_2023.pdf
Section 3(5)
To qualify under this section the child and their mother and father should have lived in
the UK (or qualifying British Overseas Territory if born after 21 May 2002) for a 3-year
period ending with the date the application is received. And the child and their parents
should be physically present in the UK or the qualifying British Overseas Territory at
the start of that period.
The child and their parents must not have been absent from the UK (or the British
Overseas Territories if appropriate) for more than 270 days during the
3-year residential period. There is no discretion to disregard absences greater than
270 days.
If the parents’ marriage or civil partnership has ended or they are legally separated,
then only the child and one parent have to satisfy the residence requirement.
Both parents must consent to the child being registered as a British citizen. If one of
the parents has died, then only the consent of the surviving parent is required.
A child registered under this section will be a British citizen otherwise than
by descent, which means they can pass on citizenship to their own children
