I can vouch for Cheakamus Lake too. Did it last summer with a 4 year old. No problem. We have been into Joffre Lakes too, and it is a step up for sure. But the big drawback for Joffre is that the lake isn't as kid friendly - rocky, big drop-offs, etc. Cheakamus has sandy beaches, etc. Way better for little kids who mainly care about playing in water. The alpine vistas or splitter granite: meh.

The other suggestion, again a step-up, and I think better than Joffre, is Marriott Basin. The hike is pretty easy (altho much harder than Cheakamus) and the lake is good for kids. And there is no stress about finding a campsite. Cheakamus gets busy so that is a concern. May will be less busy, I think. And once you have mastered Cheakamus, you can think about Marriott as a high-season alternative.