Posting your exact schedule is better, but I'm guessing you'll be using a Thalys and a TGV.

You can reservations them from NS International, at 7 Dutch stations:

https://www.nsinternational.com/nl/waar-treintickets-kopen/verkooppunten-op-ns-stations

Or via phone:

https://www.nsinternational.com/nl/waar-treintickets-kopen/telefonisch-boeken

There will be an extra fee though. The reservations will be sent via e-mail.

Alternatively, you can book a Thalys reservation here:

https://www.b-europe.com/NL/Kopen/Pass#TravelWish

(There's a 4€ fee per booking, that can be for multiple persons.)

TGV reservations can be booked via the Rail planner app, via:

More > Seat reservations > France > I have a paper pass > Book TGV trains with SNCB.

(Again, with a 4€ fee per booking.)

Do you have an Account at the Interrail Website? If You have one, add a trip and your interrail-Ticket (Type, Name an Date of birth of the passenger, …) and save your ticket (sometimes you must Save the ticket More than one time..). After this, you can search your Connection and book a Seat, When you choose your ticket an enough seats are available. I had the Same Problem a few weeks ago..