Well, I had a quick look at your Olaf cake video and honestly, you're clearly doing the best with what you have so far.
720p is probably the best, bare minimum quality you can use on YouTube nowadays.
Mic quality was clear and it doesn't look like you are using Windows Movie Maker (which is a good thing).
Only thing I'd recommend is try making a watermark for yourself, so then if anyone tries stealing your videos, you can easily take them down.