When you say hand sewn, do you mean machine or by hand stitch? In both cases, be sure to use a good quality fleece like anti-pill, polar or blizzard. Many people use a good cotton or flannel too, but I don't for the simple fact that their nails will eventually make small holes which will get bigger, and cotton just doesn't last as long as fleece. Minky fleece is not recommended because they can catch nails in it.
Hidden seams basically means sewing regular seams and make an inner pouch separate from the outer pouch, and sew them around the top together leaving a 2" or less hole to turn right side out. Make sure tabs are fleece, cut about 3/4" wide (3" long and fold over) and is cut in the direction where they don't stretch. You can close the opening with a small top stitch to close opening. Make sure stitches as tight and trim all threads.
Toys can be made from anything. Milk caps, juice lids, drill a hole in the middle thread fleece through.
Plastic bracelets(2.25" diameter), Easter eggs, tictac containers, a bag of army men , bugs or dinosaurs, barrel of monkeys, drinking straws. Any wide base container makes a great toy box. Qiffle ball with straws poked in it along with strips of fleece