Well I can tell you how they do it in Thailand but I don't think you'll like it! Involves putting a bit of chilli on the end of a chopstick and dabbing it on toddler's tongue. You can guess the reaction. Repeat everyday until they are used to it!
Obviously we never did this, my 6 year still won't touch spicy food, if I make I curry I give him, rice, a boiled egg and pappadums. I hoeping he mgiht change his mind when he's a teenager!