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."

The information in these texts is not always competely correct. The grandparents need to buy adult passes in order to get the free children's passes. 

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

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

Hi @thibcabe 

I will pass this feedback as the wording on our website is not clear enough. 

Thank you!