Without details we cannot help. Which train you’re looking to book? When are you planning to travel? From where to where?

Oh sorry forgot to add 

I am travelling sometime in early july next year, from Milano to Florence (Firenze? not sure). I am planning to travel with the Frecciorossa with Trenitalia. Thats all the details I have really.

Waaaaaaaaaaaaaaay to eeeearly to check something like that. Check again a couple of months before you go. You can buy your train tickets or reservations in Italy roughly 2 months ahead of when you'll need them.

If you want an indication of how to get there, just pick a date in a couple of days/weeks from today to see when you have trains and how long you’ll be on them. It probably won’t change much in half a year.

Yea but when I try to reserve tickets for example 20th January it still says “Ticket not available”

With me it says 10 euro for a seat. Sure you did the right steps for Italian trains? You cannot use “seat only”, doesn’t work for italian trains on ÖBB.

Find tickets → add discount → Interrail/Eurail → search again. You should find seat reservations for 10 EUR; :)

Thanks!

Found it for 50 euros thats the right thing right? (5 people)

yes 5 people x 10€ = 50€

Kids need also to pay (or I am thinking wrong?)

Yea, 3 kids (13, 11, 7)

If the kids need their own seats, then they pay the same as an adult, so that's correct.

Pass and reservations are separate and don't need to be linked.

Whether you can book the reservations via ÖBB depends on the trains involved. If you share your timetable, then people can advise. Via ÖBB you can indeed book for multiple people in one go.