It's basically a guaranteed fatality at 45mph in any weight of car - lots of towns/cities are even reducing their 30mph speed limits to 20mph for that reason.
If anything, I'd suggest doing 45mph on a road where there's the possibility of a child walking into the road faster than you could respond is irresponsible. If you're going that fast in a built-up area, you should either have a good enough view of the sides of the road that you can see the hazard developing in good time to react/break (ie, you're tracking the pedestrians alongside the road a hundred metres ahead of you), or you should be slowing down to a more appropriate speed for when you're lacking situational awareness.
I suspect this is a large part of the reasons that some countries (eg, the UK) have a hazard perception element to the driving test.
If anything, I'd suggest doing 45mph on a road where there's the possibility of a child walking into the road faster than you could respond is irresponsible. If you're going that fast in a built-up area, you should either have a good enough view of the sides of the road that you can see the hazard developing in good time to react/break (ie, you're tracking the pedestrians alongside the road a hundred metres ahead of you), or you should be slowing down to a more appropriate speed for when you're lacking situational awareness.
I suspect this is a large part of the reasons that some countries (eg, the UK) have a hazard perception element to the driving test.