Online sweater generator ,adjust skirt pattern?

Now that I am about done with the skirt, I would like to make a sweater to go along with it. I have never made a sweater before so I am here asking for more help. I would like for it to match the skirt ( would like to use the same stitches) but I don’t know how to find out how many I need to cast on and where I should start the sleeves and neck sections, I would like the sleeves to hang a little bit just enough so she can move around comfortably. Is there an easy way to learn this or do I need to do a real sweater pattern first?

Also I want to make this same skirt for my 7 year old daughter but all the sizes listed are too big for her. How do I adjust the pattern to her size?

When you are making garments is it okay to go by the pattern size or too add a inch or two to your actual size? Sorry for all the ? 's but thanks for helping me out.