Brussels to Amsterdam

It sounds like you've never been there, so I would stick to the well-known places and use trains (or sometimes buses). Both the Netherlands and Belgium have excellent train systems. I wouldn't rent a car unless you are headed to some very small, obscure towns that aren't well served by train.

There are a lot of cute small towns in the Netherlands, so between a few of them and Amsterdam you could spend your entire time there. A couple of years ago I spent a night in the town of Delft - loved it! Easy to reach by train.

Many people who go to Belgium go to Bruges and Ghent, two lovely, charming towns (Bruges is very very touristy). Some love Antwerp too but personally I didn't care for it. Brussels itself has a few fans and some interesting things to see (like the huge Grand Place square) but isn't as well regarded as the towns I mention above; it's a fairly business-like big capital city.

Otherwise, I'd pick up a guidebook or two. You can watch all of Rick Steve's PBS shows for free on YouTube so maybe view some for the Netherlands and Belgium and see what appeals to you.

Here's some good info on train travel in Europe:

https://www.seat61.com/

One way to start is with our host's (top left blue menu) free travel tips. He has pages on Belgium and on The Netherlands, as well as general advice on European travel. If this is your first trip to Europe, my opinion (I have no connection with Rick Steves) is that his books on Belgium and the Netherlands are a must read. You can buy one and read an older one at the library if you are short of cash. Both countries can be hot in August, and crowded with vacationers. Note that you have to return to Antwerp or Brussels to move on to the Netherlands, but there are several trains an hour, no advance purchase need in Belgium.

There is much more than you can see in a week between Brussels and Amsterdam. I personally like Belgium better than the Netherlands, but there are some things, like Vermeer's Delft and the Anne Frank house in Amsterdam that are absolute must-sees. You are going to have to cut out more things than you would like to. If you care about the Anne Frank House. book IMMEDIATELY, and keep trying to see if they release additional spaces. You are very late for this.

Edit: I endorse tom's suggestions just below. However, we don't know if you like art or food or history, how big your budget is, whether you have ever ridden public transportation in your life before, or how long your child can last in the umbroller.

Mechelen has a sweet Toy Museum, but a two-year old might be driven mad by toys behind display cases! I haven't been to the toy museum in Brussels. The Mechelen historical mascot is (American perspective ... ) big doll that is thrown up from a stretched blanket on festive days. Again, a two-year old doesn't care, but he can play in the "same" (mostly paved today) garden in Mechelen where Charles V played as a tot. Famous carillon school in Mechelen, market day Saturday morning.

A week isn't much and you have your 2-year old to consider. Smallish towns are indeed much better to stay, and with the excellent public transport in both countries you can easily do a day-trip. Nothing wrong with the big cities, but not with a small child.
I suggest Mechelen in Belgium, and Dordrecht and Leiden in the Netherlands. All pleasant and interesting, and without tourist crowds.
Mechelen is an agreeable historical city with everything you expect from a Flemish city, including a good market place, and close to Antwerp, Brussels, Gent (all 30 mins) and Brugge (1 hr) on very frequent trains. Dordrecht is also much overlooked: beautifully situated on a crossroads of busy rivers, some river harbours and canals, and perfect for a day-trip to the windmills of Kinderdijk (use the Waterbus river ferry!). Leiden is that typical Dutch city everybody loves, and close to The Hague (10 mins), Delft (20 mins) and even Amsterdam (40 mins), and only 20 mins from Schiphol airport, all on trains which run at least 4 times per hour.