Buy a packet of cress seeds, short names will need one packet, longer names might need two!
Mix seeds with a little sand, this will ensure they don't germinate too closely.
Ask the child to flatten an area of garden and write their name with a stick into the soil.
Spread the seed mixture in the letters.
Water lightly (a hose will likely wash away the seed so use a watering can).
In 3 days you will see the name sprout up like magic.
Cut and eat in salads of in sandwiches. Kids without gardens could grow their own name on a plate with moist cotton wool instead or in a pot filled with fine seedling mix.