Checking Eurostar reservation possiblities before booking Eurrail / Interrail Global Pass - where and how?

I would like to book a family pass but haven’t done so yet. 

We will be travelling from the UK on a particular day: and on that day staying in France with friends who will only be able to meet us at the nearest train station at a reasonably early time in the day. Ie.  don’t want to waste time or money buying the Interrail pass before we know exactly which reservations are on offer and whether it suits. 

I have gone to the generic Eurrail / Interrail help & support page about how you do this but still can’t see how I can look up which seat reservations are available. Nothing comes up when I enter our dates. Help!

I would prefer to use a helpful website than download a space and data consuming app. So no app recommendations, please. 

Queries: 

• I want to know before booking the Eurail pass whether it’s worth buying the first / second class pass. Where can I do that? And see all the available seat reservations for the different travel legs and days, before buying a pass? 
• I’d like to know if you can book kids’ seats for free via the Eurostar site, or do you need to pay for them first, then get a refund? 

Lastly - not urgently - am I missing something? When I last used an Interrail pass in the 90s, it was the easiest thing to do and way to travel ever, now it seems clunky and admin-heavy, from the initial impression I’m getting. But perhaps that’s more true for the Eurostar leg? 

Thank you for all the help you can think of!