My colleague's children can't go to the 12 birthday party.

You can consider writing a short message or sending an email to your colleagues to express your blessings and apologies.

Here is an example:

Dear XX colleague:

Thank you very much for inviting me to your child's birthday party. However, due to my work schedule, I can't attend. I hope you can understand.

On this special day, I sincerely wish your child a happy birthday, healthy growth and happiness. In the meantime, I wish your family happiness.

Thank you again for your invitation and extend my most sincere wishes to you!

In addition, you can choose a birthday gift for your colleague's child, such as books, stationery, games, toys or gift cards, so that the child can feel your care and blessing.