Set the budget before you set the alarm
Decide the amount before you go anywhere near a shop, and make it a fixed, repeating amount rather than a one-off. A weekly or monthly allowance for the hobby, coming from pocket money where possible, does more than control spending. It teaches choosing. A child with a fixed sum will genuinely deliberate between two packets and one better-known card, and that deliberation is the actual skill.
Write down what the household will and won't fund. In ours: birthdays and Christmas can include collecting, ordinary Tuesdays cannot. Queuing at dawn is a once-in-a-while adventure, not a default. Resale is a no for us at this age, because the moment a hobby becomes an investment, the joy leaves and the anxiety moves in.
The bit that actually builds the hobby
Buying is the least interesting part. The good part is the sorting session at the kitchen table: sleeves, a binder, a system that makes sense to them and not to you. Let them design the order. Alphabetical, by type, by favourite, by which ones they'd trade. Keep the collection somewhere flat and dry and out of sunlight, and get into the habit of a tidy-up before a new lot arrives.
This is also where you get the conversation. Sorting is a side-by-side activity, and side-by-side is when children talk. I have learned more about school politics over a pile of cards than over any direct question.
