The boots are a size 24 or 24.5 (They're the same, see below). If they fit, then great, why care about numbers or stickers? If they don't, try on new boots until one fits. Now, onto the questions in detail.
What do those numbers relate to?
Last: This is a measurement of the width of the boot. Most people should just want to buy what fits their foot. To an extent, wider is more comfy and narrower is higher performance.
Size: This is the size number that is probably the only number most people think about when buying footwear. It's the only number resort rental places typically ask you for. One quirk of ski boots is half sizes and whole sizes are generally identical and only the half size value is made. So your child's 24 boot is probably actually a 24.5 (which is why there is no 24 on the chart). But buyers don't always like dealing with half sizes so the store possibly sold you the boot tagged as a 24 (since it's the same). This last bit was purely speculation on my part though.
Flex: 65 is the flex or stiffness number of the boot. Higher is stiffer. It's not quite standardized across manufacturers (So a Solomon 65 =/= Nordica 65 for example), but it is pretty darn close so just think of it as equivalent. 14 year old boy's boots will be around 65ish. Adult men's resort boots are generally 85-130 depending on performance. Race boots will be significantly higher. Girls/Women's boots will be a less stiff for all ages and skill levels than boys/men's. Stiffer boots give performance typically at the expense of comfort for a non-performance skier. So most people don't need (or should want) the stiffest boot possible.
Why are there three?
We can only really speculate here. Likely it's just cheaper to manufacture one sticker for three boot models.
Why did the tag say 24 but the sticker has diff numbers?
See size section above. 24 = 24.5.