Eurostar requires a reservation and that is actually the first one you should book, as soon as you know when you want to travel. Check availability here:
https://www.b-europe.com/EN/Booking/Pass#TravelWish
If you buy a mobile pass, then you'll need to generate a pass cover number to be able to book there:
https://www.interrail.eu/en/book-reservations#/generatePassCoverNumber
Which other trains require a reservation depends on where you want to travel. Check the country information and/or a reliable planner, such as the DB website.
The obvious itinerary for a first trip would be a circular route such as London to Lille. then miss Paris with a direct TGV to Marseilles - Then via Nice and Ventimiglia to Northern Italy - maybe ending in Venice or picking one of the routes through the Alps to Switzerland, Germany (Munich) or Vienna. From there start to head home via Frankfurt and Cologne to Brussels and E* home.
Obviously you can choose any itinerary to match interests and budget and of course how old the family are.
If you were to just do Germany and Austria then, other than Eurostar, the majority of daytime trains are optional on reservations. An itinerary staying above the Alps could be Lon - Brussels - Cologne/Bonn - Rhine valley - Bavaria - Liepzig - Berlin - Brussels - Lon. Germany and Austria have extensive rail networks and frequent long distance and regional trains.
FR and IT want you to RES all their longer distance/fast trains (TGV in FR, Frecce or IC in IT)and for FR this is sometimes nightmarish. Forget ESpanan at all-nightmare for passes.
Countries like NL-BE -CH can do without any RES. In DE-CZ-AT-DK you can, but do not need to RES the longer distance trains (with the risk of-oh dear-not being able to sit closely together or even to stand a while). As you may have noted the East-EUR countries, like HU, also CZ, PL, SK etc. tend still to be quite a lot cheaper for family travel as west-EUR.
In the main site are also several proposals for specific routings to take. Use these as a base-guide only and add/delete at will. And also: about all newbees think they have to ´do the capitals´-thats nonsense-you can go anywhere trains go.