One advantage to staying in Delft is that Tram 1 goes to The Hague. If you like art museums, and have visited the Mauritshuis (NOT to be missed), for a second choice stay on Tram 1 north to World Forum stop, and walk 20 minutes to Gementeemuseum (Municipal Museum), to be renamed this October at the opening of the Monet exhibition as Kunstmuseum (Art Museum). Or take Tram 16 from Den Haag Centraal Station directly to the museum. This museum has a marvelous collection of paintings and decorative arts, in an architecturally beautiful 1930's building. There is an interactive "Chambers of Wonder" experience (in Dutch or English) (no additional fee) in the basement for children 9 and up, which utilizes laptops. Next door is Museon, with special exhibits more aimed at children, and an IMAX theater (Omniversum). There is also Fotomuseum (photography). All have separate admissions. The museum has a nice cafe, but the former museum cafe next store, Brasserie Berlage, is open for lunch, high tea, and dinner (except Mondays) and is even nicer.
Other Tram 1 alternatives include the Peace Palace ((Vredespaleis), though in my opinion, the free visitors center is not worth a visit unless you can join a tour of the building itself (check online, English tours scheduled sporadically, cost 11 euros, kids free, bring passports for the tour).
Also you can stay on Tram 1 all the way to the Scheveningen beach area (for the pier, exit at Kurhaus stop). It will be too cold to swim, but it's nice to stroll on the two-level pier and have a seafood meal, or a stroopwafel or gelato. At the end of the pier is a ferris wheel with enclosed cars (with a/c); also bungee jumping and ziplines (fun to watch even if you're staying on the ground).
Each of us purchased an ov-chipkaart at the train station for use on buses, trams, and trains: cost 7.5 euros for the card, and load euros on the card online or at the train station as needed. When you hold the card up to the reader upon entering or exiting the vehicle or train station, you will see how much is left on the card. The cards we purchased have some leftover euros, but they are good until 2024, so we hope to return to use them!