Netherlands 10 Day Itinerary Help

Delft and Utrecht are nice places to stay. Utrecht, being a university city, is the livelier of the two; Delft is more touristy.
Consider staying in Haarlem instead of Amsterdam. Amsterdam is getting very crowded, and Haarlem will be more agreeable with your young child. Amsterdam is just 20 mins away on very frequent trains, and it has a good railway connection with Schiphol airport (30 mins, with one change).
I have some doubts about Giethoorn though. Have one day more in Delft and use it as a base for Kinderdijk. A nice way getting there is the ferry (Waterbus) from Rotterdam (leaving from the Erasmusbrug). After Kinderdijk you can continue with the same ferry service to Dordrecht, an undervisited historical city, and take the train back.
A nice day trip from Utrecht is the fortified town of Naarden. Beautifully preserved and a good place for having lunch. Take the train to Naarden-Bussum, from where it's a twenty-minute walk.

"Would it be too much to add one long day trip to Brussels?"

It's not the length of the trip that's so problematic; particularly if you can do it on the Thalys with advance tickets, it's not too expensive or time-consuming. But unless you have something you specifically want to see there, Brussels itself is very uninspiring, and not worth the time or money to take time away from the Netherlands, on a short trip like yours.

That said, my brother and sister-in-law went to Brussels just for the Royal Museum of Fine Arts, and since they wanted to see specific pieces there, the trip was highly worthwhile for them. And I enjoyed my Art Nouveau tour with ARAU: http://www.arau.org/en. The Grand Place is indeed grand. Other than that - I don't have much good to say.

Kinderdijk is an absolute must. Other things to consider would be going to Aalsmeer to see the bulb auction. Make sure you check the times as it's only open for tours on certain dates/times, but this was really enjoyable for me when I visited in 2015. Also, the cheese market in Aakmaar is a fun thing to see (visit on a Friday and see the traditional market).

I am partial to this city, but 's-hertogenbosch is a great city to spend half a day in walking around and enjoying a historical city. It is one of the only cities not damaged in WW2 and you can still see the city wall. They do a great canal tour there and if you stop you MUST get a bossche bol. It's a pastry that locals go out of their way to stop in Den Bosch for. Jan de Groot is the original spot to get one, but pretty much all cafes in the city sell them. Stop in the city, walk around a bit, take the canal tour and finish with a bossche bol and a coffee :) You wont be disappointed.

It is one of the only cities not damaged in WW2
Nothing wrong with Den Bosch nor with the bossche bol, but most Dutch cities didn't suffer significant war damage.

I will be traveling my train.

Thank you for the feedback everyone! It is very helpful. I think I will change to staying in 2 places and doing trips from there.... Delft and either Amsterdam or Haarlem. I'm torn not staying in Amsterdam though being my first in the Netherlands.

tonfromleiden- May I ask why you have doubts about Giethoorn?

Sorry, I missed your questions. I hurry to get them answered.

all sides of the war had a roll in some sort of destruction of all cities in The Netherlands
As the article points out, some cities have suffered a lot and still have the scars to show it. Rotterdam, Arnhem and Nijmegen are the best known examples, but industrial cities like Almelo also got hit, and the south-west (especially Zeeland and West-Brabant) found themselves on a front-line with the obvious results.
It were mostly smaller parts of a city which suffered damage, much of it Allied bombing. In my hometown of Leiden it's a few streets around the railway station. In The Hague (Den Haag) a whole quarter was destroyed as the result of some navigational error, and a long strip was demolished to create a defense system (which, of course, wasn't of any use). It goes unnoticed by the casual visitor, either because it's very isolated or in a part you're not likely to visit.
Apart for the cities mentioned, I cannot remember any historical centre in the Netherlands which has obvious war scars, which are so apparent in Germany and Poland.

doubts about Giethoorn
It's very much out of the way (at least at Dutch standards) and somewhat awkward to reach for the lack of a train connection. Somehow it got the attention of the Japanese and/or Chinese, and it seems to be very, very busy nowadays. And I can't find it very interesting or special. The same kind of villages and landscape can be found just north of Amsterdam. Better spend that day for a day-trip along the river IJssel, with the string of historical cities: Kampen, Zwolle, Deventer and Zutphen. Or that special Naarden I already mentioned.