Here is the tutorial on how I did it to give you ideas on how you too can upcycle clothes easily and cheaply.
First grab your sweatshirt
Use your seam ripper to de-tatch the sleeves
Next grab your child that the dress needs to fit and place the de-armed sweatshirt over their head inside out as though it were a dress already.
Carefully place pins along one side of the sweatshirt to pin the sides of the sweatshirt to narrow them where neccessary to create a dress shape rather than a sack , also pin the armhole to create a smaller gap but make sure your child can still comfortably get their arm in and out of the armhole.
You may need to take the armholes in as well but I didn't need to for mine. If you need to use the same principle as for the dress sides pin in place but allow an extra 1/4 inch seam allowance for hemming the armholes.
Either cover your hands over the pins to prevent them sticking in the child when you remove the sweatshirt or draw a line where your pins are and remove them first.
Ignore my drawn line below I changed my mind on the sizing before sewing.
Fold your sweatshirt down the middle and trace your dress shape you have just pinned onto the other armhole and sweatshirt side, this ensures your dress is symmetrical.
Now use a ball point needle as your fabric is probably a bit stretchy and a stretch stitch on your sewing machine and turn your armhole raw edges over 1/4 inch from the right side to the wrong side and sew all the way around both armholes
Now sew on both sides of your sweatshirt following your guidelines you pinned earlier , a line of stitches starting at the armhole and following your line along to the side seam.
Do not cut this yet
Try the dress on the child first before any of this snippy snippy business, once you have cut it there is no going back, remember measure twice cut once.
Once you are happy that your dress fits then cut the excess fabric off the side where you have just sewn, leave about an 1/8 inch excess fabric next to your stitches and zig zag with a small zig zag stitch to neaten the edges .
And you have an easy 20 minute upcycled little girls dress.