Too much poison
is basically a type of poison, and an overdose
is any level above the recommended safe-threshold.
Thus, the simplest terms to tell a child is probably "too much poison"
chidlren's skin is thinner and thus absorbs more/faster of the pesticides, and get od much quicker either through skin or accidental oral ingestions.
Caution/prevention is key.
Consult poison control/doc..