DD got a smartphone halfway through y6 (secondhand iPhone from ebay) and DS will be getting similar soon (y6). On PAYG so means she can text/phone us when out of house but doesn't normally do anything else when not on wifi due to cost (we pay £5pm on PAYG which is enough for texts and calls, if that runs out she has to pay - though we keep an eye on it and remind her if about to run out which it rarely does). So we have a reasonable idea of what she's up to. We also have access to her phone whenever we like - agreement was if she changed the password so we couldn't get on then we would take the phone away.
Also set up with Family Sharing which means she can't download any apps without my permission (and has parental controls which prevent a lot of things being accessed but not unfortunately anything on YouTube). But, as you set the age limit on that based on their DoB, meant she couldn't have anything that required her to be 13+ (WhatsApp, Facebook, etc) until age of 13 without us lying about her age, which I refused to do. Meant she missed out on WhatsApp for first 1.5 years of secondary school, so occasionally missed a homework conversation but generally meant not distracted by the boring "hi", "hi" conversations and the mean comments which I understand were common in Y7 but now reduced. Now on WhatsApp but not asked for anything else and I'd try to discourage her but probably not say no.
They aren't allowed to be on their phones on school grounds except when teachers say they can, eg picture of homework, picture of food tech for their books, quizzes in class, early school closure due to snow. So useful for those reasons.
So I'd say get a cheap smartphone but with limits through cost and controls and knowing password.