Charlotte91 - I wouldn't stress. Wait til you have a little boy who gets an erection when you're changing him, or bathing him! Our first noticeable one was at about 2 weeks old!
He often gets them & is always naked, so the girls know all about it... (He thinks his 'big pecker' is hilarious)

Hands are always there. Just get into the habit of making sure hands are clean before & after. They do grow out of it... (Or just learn to do it in private...)