The Swiss Junior Card covers buses, trams and cable cars while Eurail doesn’t. If you’re planning to mostly use trains there’s no need to get it.
Be aware that Interlaken Ost - Lauterbrunnen, Grindelwald and above are not included in Eurail (25% discount though).
If you wish to go to Jungfraujoch, Pilatus and/or Titlis for example get the Junior card for 30 CHF (paper version only available at counters). It will be good value as the free Eurail Children pass won’t be valid. Jungfraujoch costs 20 CHF (on the cheap side while adult tickets are very expensive!), Pilatus 39 CHF and Titlis 48 CHF.
Your child doesn't need a Swiss Junior Pass if you travel with Eurail.