Half-day trip to Antwerp. Or Ghent?

They both have castles whereas Brussels and Bruges do not. I think I would go with Ghent over Antwerp.
But you can't go wrong. Antwerp is larger more urban

Darren - Ghent is a small town, quiet and quaint. There are some small shops and cafes. I found it to be more expensive than Antwerp.

Antwerp is like a small, very pretty city. There are more shops to go to and there is more to see and do. They have a pedestrian street with many shops and a huge square.

Although Antwerp has a little bit more to offer, Ghent to me is the most charming town in Belgium kind of like the fairy tale towns you read about as a kid during story time. Ghent it is Darren.

Between those two, Ghent wins. I feel there is more to do there and one can get a nice feel for the city in a few hours. The “castle” in Antwerp is really just the front of one, not an entire castle like the one in Ghent. Ghent is also closer to Bruges so you’d have more time spend sightseeing. Of Bruges, Ghent, Antwerp, and Brussels, Brussels is my least favorite, followed by Antwerp.

While it is not true that Gent is just like Bruges, it has many similarities. It is closer than Antwerp, however. A major point for Gent is the "Gent Altarpiece", which figures in "The Monument Men" movie. Antwerp is larger, and has a wider variety of museums. With children, I would choose Antwerp, and take them up to the free viewing platform on the MAS museum, and look at (at least) the Maritime floor of exhibits. Don't forget to get the kids waffles with fruit and whipped cream.

I disliked the Castle of the Counts in Gent, but I am an adult. It would appeal to young children and tweens who have never been to a castle. If you have been to a real castle, this one is lower down on the quality scale. 3 hours is a very short time to visit either city, but you would feel more like you "did Gent" than like you "did Antwerp." But any subset of Antwerp attractions makes a great day out.

A car is a liability in any Flemish city. Expect to pay at least 20 Euros for four hours parking garage time. You need a GPS because there are so many narrow, one-way streets. Rush hour traffic is awful.

The Gent Altarpiece is available seven days. Most museums close at least one day.

You did not give the month of the year. Seaside Ostend is an easy train ride beyond Bruges. Superb seafood shops in season, and a huge beach. I didn't try to swim there, so I don't know about lifeguards and current. Don't go in winter.

Thank you for the information, everybody. We aren't going until July, but I'm always an early researcher.

The key elephant in the room is "(plus driving time)".

Both Antwerp and Ghent have their parking issues, as does for that matter Bruges, but the key difference is that while it is often very hard and confusing to get to a parking place in Ghent it is often extremely bad traffic in and around Antwerp.

I hear that you are a 3 generation family and driving will be easier on the oldies and the youngens but really don't underestimate how difficult taking a car into Antwerp can be, and how it isn't a whole lot easier in Ghent.

Consider that the train goes right into the centre of Antwerp, in an absolutely jaw droppingly beautiful station, and you are in the heart of everything. And small kiddoes are free on the train, and the oldies can travel for extremely low fares on trains after 9am.

In Ghent the station is a tram ride away from the centre, but if you have a day pass on the buses in Bruges it is also valid on the trams and buses in both Antwerp and Ghent.

Ghent is closer and gives you more time to roam around.

I have only been to Antwerp in the train station but the architecture of the building is worth a look see.
I would use the train system; it is inexpensive, easy and will definitely be more relaxing than trying to drive.
You will get to see a lot of the countryside - the trains are clean, efficient and run like clockwork.