If they're all in one directory, this will create smaller versions of the pngs but leave the original files:
for i in *.png; do convert "$i" -resize 50x50 "small$i"; done
Change the 50x50 to whatever dimensions you want, and "small$i" to the output filename you want (this will just prepend "small" to the start of the original filename)
Or, if you don't want to keep the original files, this will overwrite them with the smaller ones:
for i in *.png; do convert "$i" -resize 50x50 temp.png; mv temp.png "$i"; done