If possible, you may like to try the dress on your child, safety pinning the long strap over the shorter tabs on the front to mark the ideal placement of snaps. Pictured are Birch Poppa Fasteners, a metal hammer-on snap, but many other brands of these snaps are widely available. Ensure the snaps you buy look good on both sides if you wish to make the dress reversible. Please consult the instructions on the packet you purchase for how to apply them.
(Other options: You may prefer to add a button and buttonhole. I find plastic snaps such as Kam snaps very handy. I used them on another Clothesline pattern - the Archie playsuit and made a short video on how to apply them here.)