Seventy Deadly Pills (1964)
I should be only watching my default no later than 1959 films, but I watched this because a 14 year old Sally Thomsett was in it, playing the rather frumpy Gerty, one of the gang that finds a discarded doctor's bag that has 70 deadly cyanide pills in it, which for some reason look like Smarties. Ohh Err.
From the CFF again, and not a bad film with Warren Mitchell as one of the comic robbers who stole the doctor's car, a rather nice two tone Humber Hawk Series I.
Some of the children went on to better things and there were a few faces, like Leslie Dwyer (Mr Partridge - Hi De Hi) and Harry Fowler in his usual cockney barrer boy part.
IMDb:-
A gang of children, "The Rockets" have their clubhouse (above an old garage) used by a pair of villains on the run. The villains have stolen a doctors car which had some deadly pills left in it. One of the children finds the pills and, thinking they are sweets, shares them out to the rest of the gang. The race is on for the police to find them all before the children eat any of the deadly pills.