Horn Blast Whirlwind
is a cute character: a young Air dragon of Unicorn ancestry. This character is likely to appeal to the younger set - especially those who like rainbows and unicorns. However, her base speed feels a bit sluggish and her big wings flap around a bit, making her feel more than a little awkward. Because of this, I would suggest picking up her Dragon Flight upgrade (900g) first, to allow her to move around a bit quicker. Also, consider hats or other modifiers to up her speed - and equip those as soon as possible. Horn Blast Whirlwind
's flight ability is faster than when she's just flopping around, but her turning radius is ridiculous and you can't quickly reverse direction when flying, so it doesn't work very well in close quarters.
As mentioned above, Horn Blast Whirlwind can appeal to those who like a bit more complexity / sophistication in their attacks, via the ability to chain Rainbows via the Tempest Clouds, but also can appeal to those who just want to button spam their way through. Take your pick - there's an upgrade path for that.
There are more forgiving characters for noobs and small children, but if you increase Horn Blast Whirlwind's speed a bit, she's reasonably easy to work with. Best advice for a small child (or really anyone) playing Horn Blast Whirlwind is to keep moving. Since she's not very fast, you want to keep on the move so your enemies can't surround you. If you can keep them bumbling along behind you, you can quickly turn toward them, fire off a Tempest Cloud or three and then keep running away, letting them run smack into your Tempest Clouds and, effectively, take themselves out.