Is the co-worker related to your boss? If you can't provide childcare as a team provide
parallel childcare. (stolen from co-parenting divorce training resources
)
I would recommend putting all the childrens names on post its, fold them up, put them in a bowl, then both of you draw out an equal number of names. Those are then your wards that you are responsible for 100% care of daily.
Also, leave the building for lunch. Even if just to your car. You need the quiet time physiologically. Noise stress escalates.