Between the 13 and 15, the trade-off is essentially portability vs performance respectively. Where Photoshop is concerned, the 15 can be up to 2x faster (actually more than 2x if the dedicated graphics is to be taken into account).
But in the 13's defense, I'd say... since you're getting a laptop, portability was a factor already. And if you wanted more performance, you could have gotten a desktop computer for her instead.
The 13 is inherently more portable than the 15 in every way. Plus if your daughter is not a heavy user, then the 13 is more than enough for her. Where performance is concerned, the 13 is still more powerful than the previous generations.
I'll take a wild guess and say that... it may not matter at all. What you should be concerned with, in my opinions, would be how much storage space you're getting, and how much memory (RAM) you're getting. Most video and picture users blow through storage space and memory faster than they blow through the chip.