What is the verb for saying that you are suddenly frightened or afraid?
To be startled would work:
Mr. Smith had not expected his children to be inside his room, so when he entered the room and they jumped on him he was startled.
From Dictionary.com's entry on startle:
- to disturb or agitate suddenly as by surprise or alarm.
- to cause to start involuntarily, by or as by a sudden shock.
Or to jump. One can jump in surprise, see meaning 3 here:
to move or jerk suddenly, as from surprise or shock.
Although that wouldn't look too good in your example sentence since the word "jumped" already appears there. I would rephrase it as:
Mr. Smith had not expected his children to be inside his room, so when he entered the room and they sprang on him he jumped.
Consider spook. It's often used to describe a horse being suddenly startled, but also can be applied to humans:
to make frightened or frantic
especially : to startle into violent activity (MW)
Here's an example sentence:
The news spooked investors, and stock prices fell. (TFD)
You may be looking for panicked:
To affect or be affected with panic
and panic defined as
A sudden, overpowering feeling of fear, often affecting many people at once
Consider freeze:
Become suddenly motionless or paralyzed with fear or shock
(oxforddictionaries.com)
Another single-word alternative is start:
To move suddenly because you are afraid or surprised by something
(Macmillan Dictionary)