This is something your local elected councillor should deal with - make an appointment to meet with the councillor on the problem street at a typical time of speeding (eg is it happening at rush hour) and ask them to observe.
Then follow this up with a written account of your meeting in a letter to the councillor, describing what you both saw, and asking for action to prevent motorists speeding. Make useful suggestions to the councillor, like a leaflet campaign in the area, more prominent warning signs, and a few lightning-strike speed traps.
Ask the councillor to keep you updated on the progress of the request.
Lastly, in this little suburb we had no safe crossings for schoolkids (or anyone) on a road that was notionally 30 mph but usually 40-50 mph. Council did nowt about it. The mothers made three rush-hour protests, blocking the road with a 'moving crossing' of families walking back and forth. each incident lasted less than 5 minutes ie by the time anyone in authority arrived, eveyone had faded away.
This was before smartphones so if it is a tactic you and others choose to adopt, I suggest getting yourselves some UK anonumous type masks!
We got two safe crossing points and a speed camera as a result.