Well, I am not sure why you mention " safety issues or money" as I think Europe is safer then most large American cities, and as for money, going now, or going 5 yrs from now, well, it would be MORE expensive when kids are older. They eat more, LOL>
I am taking my 12 yr old girl next year, and we took both our boys, seperately, at 14 . The boys were ( are) a little more immature and restless, so long flights and tours seemed daunting when they were younger) . My daughters personality is more easy going, so she will go younger. I guess my point is , you know your own kids best. Personally I would not have bothered at 5 , it is a lot of money to fly overseas, and my kids were just as happy to go to the States( we are Canadain) or Caribbean.
Travel overseas, time changes, jet lag, seemed a bit much at 5 to US, but, others don't find it a problem.
I went at ten to France, stayed for a couple of months , though going to " the city" ( Paris) was a bit boring, just liked playing with