To 'shell in a shell' you just work the "2dc, ch2, 2dc" in the ch2 space of the shell into which you are working.
So to shell in the next 6 shells, you just do one shell into the chain 2 space of each of the next 6 shells...like this...
In the chain 2 space of the first shell, work 2dc, ch2, 2dc.
In the chain 2 space of the second shell, work 2dc, ch2, 2dc.
In the chain 2 space of the third shell, work 2dc, ch2, 2dc.
and so on until you've worked a shell into the next 6 shells.
Then continue with the instructions until it says to work a shell into the next 11, and do the same thing only into the next 11.
This is also called 'shell over shell'.
Congrats on the new baby. This will be a lovely item for her!