I sewed these two satchels as gifts for two cousins sharing a 6th birthday party - the invitation said 'no gifts required' which translates to "Please! No more toys! The house is overflowing!" Since the boy's family does a fair bit of camping, a hiking accessory was a logical alternative.
The birthday girl has a little sister, so I made two satchels for her from a vintage bedspread; the birthday boy's was new denim. Both used lightweight cotton for their lining.
If you make one, be prepared: Other Children may ask for some of their own. (I'm already on duty for two more, minimum.)
You will need:
2 flap pieces, each 8.25" x 11", one each in the lining and outer fabric
2 bag pieces, each 12" x 19.5", one each in the lining and outer fabric
1 strap piece, 3.75" x 34", in the outer fabric
1 button or toggle 3/4" -1" wide
1 closure loop approximately 3" long (I knit an i-cord in wool for elasticity - Patons Classic Wool on 3.5mm needles, two stitches knit over and over for 20 rows)
Lay flap pieces right side to right side and slip under the needle 4.5" or less from the end of one long side, then stitch around to the same distance from the other end of the same long side, leaving at least a 2.25" gap, with a 3/8" seam allowance. Trim off corners and turn inside out, tucking in the exposed edges of the gap and pressing flat.
Stitching them separately, lay each bag piece right side to right side and stitch down the two sides with a 3/8" seam allowance. Fold the stitched piece flat with the seams in the middle, then stitch across the bottom corner to make a box bottom. Aim for about a 2.5" run of stitching, matching the length for both the liner and the outer bag.
Slip the liner inside the outer bag, wrong side to wrong side, and set aside.
There are two options for stitching the strap. Right side to right side and a line of stitching down the open edge is great if your fabric has some give (like my vintage cotton bedspread) but for something stiffer, like denim, try folding in and pressing flat about 1/4" of fabric down the length of the strap, then folding the fold over the other side and topstitching down the middle of the strap. That way, you avoid having to turn it inside out.
Fold in by 3/8" the exposed edges of both bag and liner, press, and - tucking the strap ends into place at the side seams - pin into position. Stitch down as close as you can to the outer edge while catching the lining. To reinforce the strap without adding extra rows of stitching, backstitch over it a couple of times before stitching onward.
Tie a knot in the ends of the i-cord or, in the case of a ribbon or other cord, simply fold into a loop.
Tuck just inside the opening of the flap, pin in place, and topstitch across bottom end of flap.
The flap will be too wide for the bag, but that's okay - the idea is to have a little leeway for an extra-big book or especially interesting find on the trail. Center the non-loopy end on the back of the bag - wrong side of the flap to right side of the bag - and fold in anything that extends past the straps on the sides. Pin down and topstitch into place.
It will now look something like this:
Stitch the button or toggle onto the center front; if you choose a flat button, you can wind thread around your stitching to make a shank the loop can snug up to.
And you're done! One completed bag, one very happy inquisitive child.