If you and your kids are feeling comfortable on the water and ready for a bigger-feeling trip, try camping on Students Island on Mooselookmeguntic Lake in Oquossic/Rangeley. It's a totally manageable 15-minute canoe trip to the island. Once there you'll feel like you've traveled deep into the wilderness. Each site has a picnic table and access to an outhouse, but you'll need to carry your own water to the island. You can fill water jugs at the check-in office.
There's lots of fun to be had right there on the island, or you could take short trips back to shore to play on the sandy beach. To be honest, we brought so much stuff we had to go back to the shore for a second trip, but it's such a short journey it was no problem. Like most large Maine lakes, the wind can kick up and make the water pretty choppy, so keep an eye on the weather.
We did it with kids ages 3 to 11.