I do think they should be allowed to vote. And saying they should just become citizens is ridiculous if they are EU nationals with right of free movement. Maybe they haven't (yet!) lived in GB for 5 years. Or maybe they can't afford to pay roughly £1000 per adult and £700 per child to become a citizen.
And I also think that UK citizens in other countries should be allowed to vote, even if they have lived outside the UK for more than 15 years.
My whole family are British and we have lived abroad in the EU for less than 15 years - so fortunately I'll have the right to vote. I haven't lived in this country long enough to become a citizen, so a vote for leaving will leave the family in a difficult situation. We might have to return to the UK. If we apply to become naturalised here (can't do it yet, but could in a few years) we might have to lose our British nationality, because the rules of the country we are in only let you keep another nationality if it is from an EU nation. I don't want to lose my British nationality.
If the UK leaves the EU, that will potentially affect me massively - just as it will affect those EU nationals living in the UK at the moment. The prospect that you might be told you no longer have a legal right to live in a place you currently call home, and where your children were born is unsettling. More so if you have absolutely no say in the matter yourself.
It should either be ALL UK nationals, regardless of where they are living.
Or ALL UK residents, regardless of nationality.
Ideally both.