H.P.:
Natascha:
Those heads are a bit harder to alter, but basically the same idea as with foam heads, like this...
Whynotgrl666:
Add features like you mentioned, glue on some cheap wigs, paint 'em gory...
Find someone who can juggle them, while dressed up as a blood soaked butcher, or sweet old Grandmother, to "entertain" the kiddies!
Fill a zip-lock baggie with blood, insert it into a hollowed out neck and tape it into place. Cut a small piece of the corner off, and when juggled, the necks will spurt blood all around! YAY!!!
The details needn't be perfect, the tangled, bloody hair and gore, along with the constant motion, won't allow any close scrutiny.
The spraying blood helps to keep people at a distance too!
For cutting into a styrofoam head, try a soldering iron to melt the foam. You can manuever the tip around and create a cavity larger than the opening.
If you do melt styrofoam, do it outdoors! Toxic fumes and molten goo! It wouldn't hurt to keep a bowl of ice water nearby, just in case of a napalm-like burn.
On that note... Enjoy!