I think they should have them. In fact I have to give up my phone, my tablet and my computer when the GCs visit.
DS now 19 worked out how to switch on his Granpa's computer and found the CBBC website all by himself at the age of three. Grandma was horrified, Granpa saw it as a good excuse to buy himself a new computer and give over the other one to DS and DD when they visited. Grandma bought herself a laptop, a tablet and a mini-laptop and they were seized too as soon as they were noticed.
Now we are experiencing the same with ours, we know that they will go outside and play when the sun shines, they will go on the computers when they need a bit of quiet. We also know that they are very careful with them, even the three year old. Whose favourite game is a bit like online paper doll dressing - she found it all by herself!
However, DBH is seriously considering buying them their own tablets, purely because although DGS can read well and is very good at helping DGD to find sites she wants, there is always the worry that they will come across something they shouldn't and we don't have controls on ours. Their parents only have phones that they are allowed to use, and it won't be long before DGS will need to use the internet for homework research and a larger screen would be better. We really think they need their own safe tablets if they are to stay abreast with what other children are doing.