It will depend on the laws in your jurisdiction. Though, people can generally change their name pretty easily (assuming older means 18+).

Otherwise, then it would be a matter between you and the children's father/s. If new hubby adopts the children then almost always they would be allowed to take his new name, though the procedure will depend on your local court.

Before you change your children's last name... ask them what they want!
Second of all, you can change their last name if they are under the age of 18. There is
A fee of $100 I believe and they will need a new birth certificate.