BLack Forest from Freiburg

One of the train routes from Konstanz to Freiburg goes through Titisee-Neustadt, so you can see it on the way to Freiburg.

If you go back on that route to Donaueschingen, you can pick up the Schwarzwaldbahn to Gutach and the open-air museum. Alternatively, take the main line down the Rhein to Offenburg and from there to the museum, but then the Schwarzwaldbahn would take you back to Freiburg through Titisee.

I don't know precisely how the money would work out for your family, but Freiburg hotels DO NOT offer the KONUS public transit pass that places elsewhere in the Black Forest provide to their guests. This includes the BF railroad. Perhaps you are staying with relatives or friends. I presume you know that Titisee is a VERY touristy town. It's not clear if you have one night or three nights in Freiburg from your OP. You can edit your post if the dates given are wrong.

I'm no expert, but there are many local German one-day train pass-tickets for small groups, like 4, that can save a lot of money. I don't mean advance purchase products. You might want to look into them.

My girlfriend and I got back about a month ago from 13 days in southern germany (flew into Munich then drove to rothenburg ob der tauber, then drove to Baden-Baden, then to Freiburg, then to fussen, then Salzburg and then to Munich).

Freiburg was our favorite city. We would recommend going to triberg nearby, beautiful and good people (touristy though)

By the museum, walk about 100 yards to the Gutach sommerrodelbahn ( alpine coaster ). Inexpensive, fun ride

I don't have kids, but I definitely agree on the Sommerrodelbahn at Gutach.

Here is a list of 5 things to do with kids in Freiburg (along with several other lists of 5 involving Freiburg).

Titisee is very touristy, but it's on your list of things you really want to see, go for it. I thinks kids would probably enjoy it.