Hello @msdq
The Swiss Travel Pass Flex would be a good idea for two main reasons: you can travel flexibly all day long by paying a fixed price and your child could travel for free (valid for children under 16 years of age accompanied by at least one parent) with the Swiss Family Card.
The route Grindelwald - Disentis/Muster is actually included in the Swiss Travel Pass. Mountain destinations beyond Grindelwad such as Kleine Scheidegg or Jungfraujoch are not included (but there’s a discount). You can check the validity map here.
Another option would be the Saver Day Pass, to be purchased in advance for every travelling day. In this case, prices are subject to change and there is no refund. Children between 6 and 16 years old get reduced prices. The validity area is the same of the Swiss Travel Pass, but without the discounts on some special route (for example, from Grindelwald Terminal to Kleine Scheidegg).
I hope it is a bit clearer now. My colleagues at the counter would be anyway happy to help you at your arrival.
I wish you a lovely day and an amazing stay in beautiful Switzerland,
Chiara_S