Ste Cergue is nice, very small but quaint, you can walk around a bit, make a little hike from there.

We have just been to Yverdon les Bains a few weeks ago with our 3yr old and she had a blast. One does not really notice the mountains missing when constantly supervising a kiddo, haha. It is very pleasant there, the water is 34 degrees if I remember well, they have swimming aids to help the wee children float.

One nice trip from Geneva we like to repeat every now and then is beautiful Annecy, the view of the hills there is amazing. The only bitch is to find a parking spot there during the weekends, but maybe you will be luckier this time of the year. Check out the little food stores with local delicacies, yummy.

I can also recommend a boat trip to Yvoire across the lake, fancy fish restaurants and a nice cute medieval French mini town. It gets windy there from the lake, I'd make sure to wear enough (your inlaws could be cold..).