Here are some options, depending on the context:
Formal:
* Twin: If the two children are born at the same time.
* Siblings: If the children share one or both parents.
Informal:
* Kids: A general term for children.
* Little ones: A term that emphasizes their youthfulness.
* The two of them: A neutral phrase that avoids specifying a relationship.
Other options:
* Pair of children: If you want to emphasize the quantity.
* Two offspring: If you are being more formal or scientific.
The best term to use will depend on the specific context of your writing or speech.