Anything with a Core i3 and above will be fine for Photoshop, provided you have at least 8 GB RAM. For really really heavy stuff you may want an i5 with a dedicated gpu. But for normal stuff, even a Core M3 6th Gen will be fine.
Specifically
CPU/Motherboard - Something based on a 6th/7th gen Intel 'i' or 'm' series. Do NOT go for Pentium or Celeron branded CPUs. They suck for anything heavier than web browsing and minecraft. With or without a dedicated GPU - ok either way (provided your niece is not a hardcore gamer or something!)
RAM: AT LEAST 4 GB, recommended 8 to 16GBS.
Harddrive - Solid State makes a HUGE difference to general performance. People underestimate just how important storage can be in day to day operations. An i3 with an SSD will kick the ass of an i7 with a mechanical drive, easy. Boot times will be MUCH faster as well. This is one area where I never compromise. However, SSD equipped machines are usually on the expensive side and have smaller capacities, so if you have budgetary constraints, they might not work for you.
Display - As an art student, color accuracy and gamut is very important i would assume, so go for an IPS monitor with AT LEAST 1080p resolution. Avoid TN displays like plague.
Rest of the specs are not that important(battery perhaps) so you can look around and pick one that suits your budget and requirements.