Finally I can offload the experience gleaned from the living nightmare that was setting up a new tablet PC which I now use to do proper grown-up work on! And not just rude sketches of polar bears, oh no. I am a grown-ass adult and that was 900 balloons well-spent.
If they're going to be doing work on the sofa, in their bedroom, in the car etc, then a tablet PC is the easiest and most stable option for portable use, but
expensive. Apple products are the most spendy, obviously, but also the most stable, though once they break down, you're shit outta luck until they send you a new one and you may or may not have lost your work files and other data depending on how diligently you back-up (if at all). You're also locked into subscription-based services if you want any half-decent apps to work with. You might get lucky with a third-party Android build if you look around - just learn the difference between "capacitive" and "resistive" screens and stylus' first - but your art app options are pretty limited and chances are high you'll have to use a freeware option like Glimpse, which is perfectly good, but like most freeware apps possesses a steep learning curve and some features work counter-intuitively, so might be frustrating for younger users.
A refurbished Surface from the local
stolen goods fence CEX or Cash Converters might be a good option, if only because they usually have good warranties - CEX guarantees electronic goods up to 24 months as standard, which is not bad for something with a relatively high failure rate even with factory-sealed models. It also runs Windows, which for all the stick it gets - including from myself - is still the OS that runs most software, and is easier to troubleshoot via Google searches without having to deep-dive on tech forums.
If you do opt for a tablet PC, be warned that most modern art apps like the ubiquitous Clip Studio are subscription-based, while most older versions of apps are incompatible with pressure sensitivity on newer hardware, but Manga Studio 5 (Clip Studio before it rebranded) and Adobe CS6 will work just fine even with a third-party stylus (which I would recommend for the lower price), and you can probably find a second-hand physical copy that can be transferred to the tablet via a memory card.
If they're going to be working in a set area and nowhere else, then a laptop/PC with a plug-in tablet is fine, and laughably cheap to get started - and upgrade - if you're prepared to go looking for artists offloading equipment they no longer use, while buying replacement CPUs, RAM or video cards from your local electronics pawnboker or Ebay. Seeing as this is a laughably cheap and forgiving route to travel, a diligent parent should possibly make every effort to impress on their child that REAL artists have dedicated studios where they do their work. Throw in some buzzwords and phrases like "workflow stability", you'll be grand.
If you go the upgraded PC route, don't be fooled into thinking you need a tablet with a screen to work on - the cheap-ass tablets that are basically just rubber mats and cost less than 20 quid if you shop around will do more or less the same job as a tablet PC or one of the fancy HD widescreen Wacom buggers, especially once you factor in the software you'll be running on it and the ability levels of the user. Pro artists used these for literally decades and switching to a screen didn't make much difference to their output. You can buy a dirt cheap one of these ahead of any other larger expenses and get started on any PC just by plugging it in, so it might be worth testing the waters first - a quick look on Amazon reveals some perfectly good starter options
like this one for 28 quid.