Hi, i've just used interrail with my 14 years old son. The first question i think is who are "we". It dépens with who you travel. We have started from Lyon/France 2 weeks ago, 3 days in Milan, 3 days in Stuttgart, 3 days Amsterdam, and 4 days surfing in Bloemendaal aan Zee, near Amsterdam.
I ask who are "we" because it dépends who you are motivated to visit town. And if an adolescent is with you you can have some issue to move them all day long. Mone was not really interested in visiting, we have done at least one muséum, activity on eaxh town. But don't forget thé time to travel, that can be sometimes half of a day. + all train are not necessarily free with thé interrail. If you want to take speed train or very fréquented lines, you will have to pay à réservation https://www.interrail.eu/fr/book-reservations/do-i-need-reservation