senior pass, free child pass?

Yes this what I'd do : get 2 adult passes. Otherwise how would grandparents travel with their grandchildren I'm thinking... There's nothing preventing you to do that.

Here's more info about seat reservations if needed :

https://www.seat61.com/interrail-and-eurail-reservations.htm

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

The Eurostar is the main issue. There's a passholder quota and some trains can sell out weeks in advance. Some dates around Christmas are already full (very busy period).

EDIT : Mmmmhh you should be able to buy a Senior pass + a Child pass, it's claimed here : https://www.interrail.eu/en/magazine/did-you-know/rail-holiday-of-a-lifetime-with-interrail

"Large families are rewarded with extra savings. Let’s say your family trip includes two parents and two grandparents; that means up to eight children in the family can tag along for free."

Then it needs to be corrected/detailed @Mukhammad @Camilo. 

It's fine to buy adult passes for grandparents but it should be mentioned somewhere.