Couldn't agree more @REI-ShanaT. I'm a SoCal native and have hiked/backpacked the PCT and generally throughout much of the state. Based on your parameters, Mosquito Flat/Little Lakes Valley would be my top recommendation. My dad took me on my first backpacking trip there when I was in 2nd grade. Chickenfoot Lake is about 3.5 miles in and is really beautiful. The hike in has minimal elevation change so it's not too difficult for youngsters but still provides amazing views. 

If you want something a little more challenging, I like @Philreedshikes suggestion. Or alternatively, you should look at Charlotte Lake via Kearsarge Pass (from Onion Valley trailhead it's about 16 miles rt with significant elevation change--not easy) or north fork of Big Pine Creek (similar distance with less climbing, though still challenging), which has a series of amazing turquiose lakes that used to be fed by now-melted 😞 glaciers (aka the Pinnacles). 

Hope you have a great time!

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