The Singapore government grants Citizenship to the children of Singapore citizens in three different ways.
- Citizenship by birth – if the child was born in Singapore and at least one of the parent was Singapore Citizen at the time of birth of the child
- Citizenship by descent – if the child was born outside Singapore but either the father or the mother of the child is a Singapore citizen, and the child did not acquire the Citizenship by the birth of the country the child was born in, then citizenship by descent can be applied in Singapore
- Citizenship by registration of minors – this is only applicable when the child is not older than 21 years, was born to a Singapore citizen, and is residing in Singapore
In your case, you have indicated that your husband was a citizen, however, there is no mention about your status. Citizenship by descent has to be applied within one year from the birth of the child.
Since both of your children are above the age of 21, they do not qualify for any of the above schemes. That means they will have to apply for Citizenship on their own merits. They will have to be first PR in Singapore and after being PR for at least two years they will qualify to apply for Singapore citizenship.