well, it does depend on what you shoot and how fast it moves, if it's adults or older children then it wouldn't be much different to a zoom, obviously younger kids running around is going to be much more moving around.
but you have to balance it out, a 28-75 lens isn't going to give you a huge amount of usable portrait focal length anyway, but in any case it will be a compromise - with the zoom you get convenience, with the prime you get ultimate image quality, better bokeh, more dof control etc etc.
my kit bag comprises of 3 primes and 1 zoom. a prime is on my camera much of the time. i only tend to use the zoom when i'm going somewhere like a zoo or on holiday where i won't always have much scope for zooming with my feet or i don't want to carry too much kit.
personally, i don't find myself running around all that much - i have two young kids and even with a zoom you are moving around to get the best framing so i don't think that should really be a deciding factor unless you have limited mobility.
and if shooting portraits is going to be the main use of the lens, i don't think there are many who'd argue the 85 1.8 is a great lens and will do the job just dandy.