Suzanne, Lauterbrunnen is great for kids. I brought each of mine there and they all want to go back as adults.
In my experience, only half the days will be "great". Other days may be misty, overcast, rainy, or gray. However, one great day makes up for 3 poor ones. I say that because if your first afternoon happens to be that one sunny day, you'd hate to waste it. Both falls you mention can be easily done and are just fine on a misty day. So I'd suggest that if you arrive and it's sunny, grab some snacks from the Coop market across from the Lauterbrunnen train station and get going! The hikes I mention below are all easy, each takes about 2-3 hours (though it takes an hour each way to get to Grindelwald/First), and so you really could do 2 in a day if it's a great day.
I'll copy my overview of the best (and easy) trails from another thread:
There are loads of great trails in the area, but 3 that I consider "classic",
and they follow the 3 great ridges. The first goes
from Laut to Grutschalp to Murren. From Murren, you can go down to Gimmelwald or you
can hike uphill on a diagonal from the
Murren cablecar station, and eventually end up at the restaurant at
Spielbodalp. There you can get a drink and sit at a picnic table and watch the
cows. You can see that trail and others here:
http://www.gimmelwald.com/pics/gimmelwald/hikemap.jpg
Laut. to Murren is about 2 hours, another hour to Spielbodenalp. Take the Murren to Grutschalp train back, and that takes about 20 min.
The second goes from Mannlichen to Kleine Scheidegg. The walk itself is
about 2 hours, and we save this for the sunniest day, because of the panoramic
views. (Leave in the morning--most afternoons the peaks cloud up!) Take the
train from Lauterbrunnen to Wengen, then a 5 min walk to the cable car
station, where you take the cable car up to Mannlichen. Once up, you can look
left at a 30 min up and back trail to a lookout point. If you look right
there is the main trail that leads toward the Jungfrau mountain, which takes
1.5 hrs and you end up in Kleine Scheidegg, where you can catch a train down
to Wengen and finally Lauterbrunnen. Great panoramic views all along the
route. This trail is so easy you could do it in a wheelchair.
It takes about 20 min train up to Wengen, another 20 min cable car to Mannlichen, 1.5 hr hike to Kleine Scheidegg, 40 min train down to Lauterbrunnen.
The 3rd classic trail is from First to Bachalpsee. Take the Lauterbrunnen
train down to Zweilutschinen. Get off to switch to the train going up to
Grindelwald. Once in Grindelwald, walk about 20 min further into town to get
to the cablecar station, where you take it all the way up to First. From there
walk south to Bachalpsee. There are trail signs, and it is about 45 min each
way. On a sunny day, you can get great pictures of the mountains reflected in
Bachalpsee, which is a small lake. Just Google image "Bachalpsee" and you'll
see what I mean.
Finally, if you get a foggy or misty day, you can walk from Lauterbrunnen to Trummelbach Falls, which is about an hour each way (or take the PostBus). Google image it to see why Trummelbach Falls are worth seeing, and the nice thing is you can actually see them just fine on a dreary and foggy day.
Allen
