Masks should be made in three layers
Cotton with 180 thread count is perfect for the two outer layers. If you buy polycotton the more cotton percentage the better. If you lift the material up to the sun or light, the less light allowed through the better the weave tightness or thread count. The inner layer against the face can be a less thick weave. Try the candle test to check for the effectiveness of the material. If you can blow out a candle with the mask then the fabric is not ideal nor effective. Some have recommended flannel fabric which is ideal during winter.
You should add a filter between the two layers as a third layer. Spunbond polypropylene (60gsm) is a non-woven fabric made from fine continuous filaments of polypropylene that are bonded together using a heat process. It is safe, economical, extremely durable, and flexible, promotes maximum airflow, and is reusable. A “coffee filter” or a “vacuum cleaner bag” is not recommended as a filter as these may disintegrate with washing.
A tie can work with elastic, ribbon, or simple material ties. It can loop over the ears or be tied at the back. The important thing is comfort and tightness of fit. The easier option for kids may be to have a looped elastic around the head or a drawstring with button adjuster, stitched to the lower end of the mask, and a loop at the top. A single drawstring attached to both sides is also very simple and easy. The button adjuster allows you to tighten and loosen the mask when needed.