Meysan Lake trail is steeper - more of an exercise. The lake is a nice goal, but not a big whoop. Except kids love to throw rocks at water.
The trail up to Kearsarge Pass from Onion Valley is moderate and only really is discouraging the last 1/4 mile or so when you see it going up, up ,UP and disappearing over the pass. Just be prepared to take the whip to the kids to get them over it:). You go around a large rock outcropping to a relatively flat spot just before you start up the last pitch. It is temping to die there. Don't unless you really do need toe tags. There are a couple of really nice lakes just off the trail on the way. Except for the lakes and drainage, I don't remember there being water on the trail however. It will be a long day for all of you.
Before you go there read up on the area so you can WOW the kids. There is a gold mine up there that is still full of gold, but is now in a Wilderness and is crammed full of ice - avalanche finally closed it early after it was opened back in the ol' days. The city named after a ship and not an event. A very large stamp mill and small town up there was completely wiped out by avalanche. New road finished the job. Much more available on the net.
I'm guessing you could leave early on the trail and get into Kearsarge Lakes for lunch and back well before dinner time. If the group hasn't had a hike like this before, it will be a thing they will remember for a long time. The view from 12,300 feet is as awesome as is the feeling of accomplishment. Take a map so you can point out the big hills you will see south, west, north. There could be snow up there. Enough for sno-cones. Take syrup.
The trail switchbacks up a series of 'steps' caused by receding glaciers. It is disconcerting to watch the parking lot disappear going up and reappear, upon return, at such an alarmingly slow rate.
Before you go down to Kearsarge Lakes remember you have to go up a fairly steep part of the trail near the pass on return. If some of your charges had a real struggle to get to the pass, you might have lunch at the pass and head on back down when people get bored...or cold (take jackets). I've taken < 10 yo on overnights to K Lakes and beyond.
I suspect those kids will be a lot happier the next day than will you.
If you don't mind not being at the portal area, there is a really nice car camp area a short ways in on the road up to Taboose pass. It is the first left turn going north out of Independence.
