1) Is the profile of the SP DSA keys suitable for my Quick Fire Rapid?
The profile of the keys doesn't usually matter functionally, as it's just the shape of the keycap which sits on top of the switches. It makes the keyboard feel different to type on, but whether you like or dislike different profiles is down to preference. Since the Quick Fire Rapid has a standard layout, all you will need to make sure of is that the keycap set is designed for Cherry MX switches, which almost all keysets are.
But I noticed you're in the UK, so you may need to also get your hands on the keycaps necessary for an ISO layout (in which the enter key is shaped like a backwards P). If this is the case then you may run in to a few more troubles. If the enter key is a rectangle, like the backspace & shift keys, then things will be a little simpler.
2) What the hell is a Child Deal?!
When buying keycap sets, like EVE for example, there is a standard or base kit which usually includes all the keycaps necessary to cover a Tenkeyless (without numpad) keyboard. From there you can also buy whichever extra keycaps you may need - which may be different sizes depending on your keyboard and layout. This is done because selling every keycap in a set to every single buyer would be very expensive, and lead to a lot of unsued keycaps.
I hope this answered all of your questions, if you're still curious about anything then please feel free to ask