Only certain pets have the possibility of developing spritely. A couple of examples that I know of are Kraken, Spinyfish, Starfish, and Sea Dragon - but there are many more.
If you are reluctant to buy any of these pets using crowns, you might consider hatching meet ups.
Word to the wise though, not everyone may be willing to hatch with your kid's pets if they are low ranked pets with undesirable traits.
The pet you hatch with should already have spritely as a trait. The best way to see this is to click on a wizard and then on the little red arrow in the lower left hand corner of the information box. On the next screen pop-up, click on the upper arrows to navigate to the equipment page - there you can hover over their equiped pet and see the displayed traits.
Hatching with a pet that has spritely does not automaticaly gaurentee your new pet will have it or that they will develop it (I had to train my spinyfish to ancient before it developed spritely).
I ended up hatching and training about 8 Spinyfish and 4 Starfish before I got the spritely spell - so don't give up and good luck!
