I would say upfront this isn't my children's experience but my own. I was frustrated with the amount of time I spent on social media and quit a few years ago and am current using a feature phone. For the most part I'm happy with my decision. I'm not a particularly gregarious or extrovert person, but even I noticed a significant drop in invitations to parties and social events sent out via Facebook and the like. If you're not on Facebook, people don't take the time to send you a special message via other channels unless it's a very special occasion or you're really close. You could argue that's better, but the feeling you're missing out on something is undeniable.
Could it be that your children are missing out on things because of the lack of a smartphone and instant messaging like iMessage? Sure they can use a desktop, but WhatsApp and iMessage don't work there, the two most prevalent IM services, and their social group in their year most likely are conversing via that. Have they broached the subject with you?
My children are between 2 and 5 and these are questions I'm grappling with at the moment. I'd love to know other's experiences, particularly yours, because I honestly don't know whether smartphones are a good idea or not.