Hi there, absolutely no need to panic! It is unlikely you will be refused based on a mistake.
Two kids under 3 is enough to make anyone forget something!
For good order, I would write a letter and submit it, with Birth Certs explaining you forgot to add them. UKVI are reasonable people - if you fulfil the criteria, which you do, you will get the visa. They are not going to penalise you for a small error made in good faith.
Good luck.