The Singapore government grants Citizenship to the children of Singapore citizens in three different ways.
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.