You could take a seat in the Restaurant onboard of the train, you have to buy something.

Look at bahn.de if all seats are full, interrail.eu often has some bugs.

Keep in mind that on ICE trains there are special compartments for families. Bookable only via Deutsche Bahn.

Possible is that people do not show up. If someone does not show up after 15 minutes the seat is yours. There are small displays over the seats so you know where there are free seats. 

Reservations are optional on that route so you could always board the train and look for free seats. There are multiple trains per day so I'm sure they're not all sold out.

What is the date and departure time ?

Optional reservations cost :

- 4,50€ per person

- 9€ per family

What date and time do you want to travel?

Thx for help - travelling this afternoon!! Trains packed due to cancellations so we’re trying to avoid standing with kids for 4hrs are there BahnBonus seats we can use? How will we know if a seat is free? 

It looks a bit like last summer with chaos all over indeed... start of the Easter holidays :/

This connection still has 6 seats but book now :

- Frankfurt 14:20 - Stuttgart 15:53

- Stuttgart - Zurich 16:13 - 19:23

Otherwise take the RE to Mannheim at 13:12 but you'll surely stand up

There are indications over the seats whether the seat is occupied or not btw. But the train to Stuttgart looks the best option. Lots of seats between Stuttgart and Zurich, even tables of 4 :)

Let us know how it goes

EDIT 13:19 4 seats left now between Frankfurt and Stuttgart

Did you get to Zurich in the end ?

Yes thanks everyone! We fudged it on first train by taking membership seats for German rail passes then booked 4 seats for half way through journey but managed to get seats (by hovering!!) but it was very busy!