How do you get those legs strong?

Hi everyone. I've been using FB strength videos and love them but my poor legs get tired when those pesky squats, burpees, fly jacks, and the like are part of the workout. Like, can't finish them tired. What have you all found to get those legs in shape? Are there any beginner videos from FB you can recommend?

Also could work on strengthening my abs which after 5 kiddos are a bit lacking in strength as well. Beginner suggestions or FB videos as well?