We did this transfer at Interlaken Ost to Lauterbrunnen or Grindelwald, or vice-versa, 4 times last summer. It is easy but you do have to go down the ramp or steps into the corridor under the tracks to change platforms.
The train from Spiez comes into Interlaken Ost at one of the higher-numbered platforms, like 5, 6, 7, or 8. The local train to Lauterbrunnen and Grindelwald (same train; they split it at an intermediate stop, Zweilütschinen) departs from Platform 2. For Grindelwald, you want section 2B—it is clearly marked on the platform. The 2A section of the platform is for Lauterbrunnen (and Wengen and Mürren). If memory serves, 2B is the back half of the train, but check the signs to confirm
Many of the people getting off the train at Interlaken Ost will be going to Platform 2 for Grindelwald and Lauterbrunnen. Some will be going on to Luzern, so they will want Platform 4. But everyone will need to go down the ramp or steps to the corridor under the tracks, so you can follow the crowd into that corridor, and then go up the ramp to Platform 2— you will be heading toward the station, if that helps.
If the train is crowded it can seem a bit confusing, but just remember you must go down to the corridor under the tracks, and then come up at Platform 2. Then check the signs for 2B. With parents and lots of luggage, you may be slower than others, but as long as you proceed directly to the platform you should make it. You may or may not get seats but it is a short journey if you have to stand. And if you do not make it, or the train is too crowded for you to board, there should be another along shortly. The trains departing Platform 2 only go to Grindelwald and Lauterbrunnen, and they are always the same—-2B for Grindelwald.
