Cut two pieces of fabric approx 30cm x 18cm - I used a cotton outer and nylon lining (apparently food safe)
Pin velcro (approx 15cm) as in pic
Carefully stitch close to the edge - I use stitch width 0 and length 2
Pin right sides together (ie the velcro will stick together). Stitch down each side leaving about a 3cm gap on one side (you'll need this to turn the bag right side out later)
Turn lining right side out and set aside
Place the lining (right side out) inside the outer
Pin together making sure seams are aligned nicely
I suggest if the arm of your sewing machine is a bit wide to make the snack bag a little bit wider - it was a really tight squeeze on mine. Stitch all the way round.
Your work should look like this so far
Turn right side out by pushing through the gap you left in the lining
Stitch the opening closed - I used stitch width 0, length 2 but you could also hand stitch this closed
Push the lining into the outer and you're done. For children's snack bags I recommend stitching a seam along the top just to reinforce.