Day trip from Amsterdam to Delft and Kinderdijk

yes, that's quite doable.

I would combine Kinderdijk with Dordrecht, to be reached with the same Waterbus. A much more interesting city than Delft, if only for its wonderful position on a crossroads of rivers.
But a combination with Delft is very well possible. I'd first go to Kinderdijk, and then track back to Delft. You might find the schedule of the alternative ferry service more convenient. That one involve a short trip with the so-called Driehoeksveer (»Triangle ferry«), where you can pick up the ferry to Rotterdam. This page explains it all.

Thank you for your replies! We were planning on renting bicycles in Rotterdam and taking them on the ferry. Is it better to rent bicycles in Kinderdijk? We will be there in July, so we weren't sure whether we need to make a reservations for the bicycles.

I don't see much use for bicycles at Kinderdijk itself. Unless, of course, you want to make a longer tour. It's a nice area with good bicycle paths (see this map) and you'll like villages such as Oud-Alblas.
Renting in Rotterdam might be the safer way.

The bike and walking paths you take through the windmills locations are not that long. It was a nice walk but not far enough to rent a bike. Bikes are available for rent across from the waterbus dock in Kinderdijk.