My son has a fish allergy. We had skin prick and blood test. He had these at 26 months.
It was fine. The consultant made it fun, she drew on his arm and made a story out of it. He had a saline drop, a histamine drop (controls) then he had tests for salmon and cod.
Once she'd done this, she scratched through the drops with the top of a scalpel (sounds worse than it is, he didn't even flinch) and then waited for the results. He didn't react to any of them, even the histamine, which means the test didn't work. Apparently about 30% of people don't react to skin prick tests.
However, she confirmed that he had the allergy, as the history was so conclusive.
His blood test was fine too. The phlebotomist was lovely and distracted him with toys and stickers.
I was DREADING it, but he was absolutely fine.
Anyway, I hope this helps.