They're between the ages of 8 and 10, though, maybe they're 7...
Kids' ages are really tough to remember and super important to get right. Could you imagine the reaction a 6 year old would give if they got a birthday card celebrating their 5th birthday? Martha Stewart would wilt under that disappointment! It's best just to play it safe - no specific age, no age-dependent characters, just special birthday treats and toys to celebrate their special day.