They can still call 911 by holding down the 9 key:

Authorities tracked down a 4-year-old girl who called 911 nearly 300 times last month by offering to deliver McDonald’s to her suburban Chicago apartment.
Unbeknownst to her mother, the girl used a deactivated cell phone to call dispatchers 287 times in June-sometimes as often as 20 times a shift.
Dispatchers heard the child’s voice but could only track the phone’s signal to the apartment complex.
[…]
Under federal law, deactivated cell phones still must be able to access 911. Many deactivated phones will contact an emergency call center if the user holds down the nine key.