>>703
To wash each other's backs.

Some of the baby words end with "っこ".
にらめっこ - To look at each other
かけっこ - To run together
Basically, "っこ" adds a meaning of do something together.

As you probably guessed, there is another "っこ" that means a kid or a girl,
such as めがねっこ (a girl with glasses).