Thank you so much for the gift of diapers! I know we will certainly put them to good use! We are so grateful that you thought of us during this exciting time.
Or something of that nature is what I would go with.
"thank you so much for your present, they will be very useful"
I'd make some kind of joke. Maybe go with something like, "Dear Co-workers, I really appreciate the shower you had for the baby and me. It means a lot to me that I work with such a great group of people. Thank you so much for the diapers! Unfortunately, I think they'll come in handy. You'll have to ask DH about that! After all, boys are in charge of gross things!"
Something like that. At least the 'unfortunately' part. Maybe draw a little smiley face in there, too.
It depends on the crowd. My co-workers would appreciate that kind of joke, but I know people in other depts that would not.