Okay, well, before I even watch the video, you lose points for asking critique against a 7 year old.
Alright, round 1.
Honestly, a lot (if not all) of those deaths shouldn't have happened. Both of you need lessons in recovery.
You, personally, need lessons in approaching. I know it worked on your brother, but just running in blindly isn't the best idea. It seemed like you were just running in trying to just do something, and get out of there. It was a random match. Lots of pointless death.
Basically, from round 1, I gather you need to be in control of Sheik. Know what moves you want to do, and when you want to do them.
You got some good D-smashes in but all they did was toss him around. Learn to position yourself when you do them to knock HIM off stage. He was at high %ages, so you could have edge-guarded well.
Round 2.
Well, considering all 3 times you died it was because of bad recovery (which I can also say to your brother), I think you need to practice your edge-guarding/recovering game. Marth has a faster recovery than you. THIS is when you should have been Vanish Gliding.
Position yourself so even if he's edge-hogging, you will land on the stage. Let her drop a bit and move away from the stage, then recover from there. You'd be surprised at how far she can go.
You could have footstooled him quite easily when he was at the edge. Practice doing that.
Anyway, on a more positive note, you clearly have control over the Boost Smash, AND you can apply it in the game, which is really good. Waiting for the opportunity instead of running in and hoping it's there is what you need to work on. Overall, it's a good Sheik you have there, but I think you need to play some better players.