There are a number of different types and sizes used to make art cards. Traditional ATCs are two-inch by three-inch cards, while post cards can be five inches by seven inches or four inches by six inches. Folding cards can be the size and shape you prefer, although for large cards, use thicker, stronger paper to avoid the card getting floppy.
Art cards are made out of a variety of materials but care should be taken to use appropriate materials. Ideally, you will use a thin card stock or thick canvas paper, if you prefer a textured look. Many artists prefer to make cards out of recycled materials instead of buying expensive pre-made cards. You can use this to give your child a lesson on recycling and sustainability as well as art. Cereal and snacks box cardboard and poster board make ideal card stock.
A unique look for your cards is to cut them into fun shapes, hearts for Valentine's day, for example but more interesting option is to make them three-dimensional (3-D). If you use a thinner card stock or paper in a variety of colors, you can cut each sheet into a different shape and glue them one on top of the other. The best versions of this project look like topographical maps. You can also use art cards to make pop-up scenes.
While art cards can be made with any supplies that work on paper, some things just work better. Avoid using water colors and oil paints since card stock absorbs the liquid. Acrylic works better and covers painted cereals box cards. If you use decorative touches like beads and bauble, look for ones with flat backs. Using a pencil or charcoal sealant spray to keep the image from smudging and a glue to keep it all together.
Art cards are an ideal way to learn artistic skills that may be cost-prohibitive in a larger size. Miniature encaustic paintings can be a great way to practice and test how much wait the wax can hold if you like to suspend cute thing like keys in the wax. You can also try new styles, like pop art or Japanese ukiyo-e (wood block prints) since the small size makes it harder to see minor mistakes.