day or half day trip from florence to pisa

The best way is by train (travel time from 50min to 75 min depending on train, which can be Regionale or Regionale Veloce, i.e. regional or fast regional).

Pisa has only one piazza to see (Piazza Duomo) where you can see the famous leaning bell tower, the cathedral, the baptistery, and if you want, the historical monumental cemetery and museum. Some might say half day is more than enough for everything. Other people (including myself) might say a couple of hours is more than enough for Pisa.

Since Lucca is very close to Pisa, and it's even more beautiful, in my opinion, you might consider the short ride to Lucca as well (23 min by train) and visit the other half day there, if your kids can handle it.

You don't need to come back to Pisa to return to Florence, you can come back directly from Lucca, although it's a different railway.

So you could do Florence > Pisa > Lucca > Florence, in clockwise direction, or the exact opposite, in counterclockwise direction. It's up to you, however I think Lucca deserves a couple of hours more than Pisa.

The main reason to visit Pisa will turn out to be the less impressive. The adjacent cathedral and baptistery are certainly worth a visit. Pisa has a train station - S. Rossore - which is less than ten minutes walk from the tower.
Lucca is indeed a nice city. The kids will like the walls. Much, much quieter than the madhouse which Florence can be.

We did Pisa and Lucca from Florence, by train, in one day. We, by far, enjoyed Lucca the most. Piazza Duomo in Pisa is wall-to-wall tourists trying to get the perfect "holding up the tower" picture or wielding selfie-sticks to the complete disregard to everyone around them. Lucca was absolutely sublime.

Lucca is great for kids. You can walk or ride bikes along the wall (don't know their ages), and there are playgrounds for younger ones. It is not very crowded, more laid-back.

I'd do Pisa first and as early as you can. You will avoid the crowds that only increase as the day goes on. Allow more time for Lucca.

You can easily see both Pisa and Lucca in one day. I think your kids would enjoy seeing/climbing the leaning tower as it is iconic. Our 2 boys were e!em & Jr high age when we went to Pisa and they loved climbing the tower. Lucca is very nice and it is fun to walk or bike the wall. Buon viaggio!

I agree with everyone so far. Go to Pisa early (central train station is long walk to the Campo dei Miracoli, go to San Rossore). You can tour it on your own. Get out of Pisa before lunch, train to Lucca, have lunch there (put together a picnic to eat on the wall, or choose one of many good restaurants in town).

There is a tower with a tree on top that some of you may want to climb.