So you've opened the XML file, but you've been to confused to find anything, right?
It's not so complicated, like it looks at the first time

.
In this XML file, every unit has an entry, with nearly all values, which are needed for the unit, like strength, movement, etc.
Search for UNIT_SETTLER, this will lead you to the entry for the settler. All, what is mentioned until the enxt entry of this type belongs to the settler.
If you look a bit closer at the data, you'll find this:
iGroupSize says, that the unit consists graphically of 4 units.
If you look further, you'll notice, that 1 one the 4 units is a male settler, one female settler and 2 children.
And yes, that's it. So you create multi units. But you have to do it yourself, because there's no specific mod for this.