Learn English – A word for someone who everyone is scared of

The compulsive need to exolain everything in the minutest detail is contained in the rhetorical term Epexegesis

epexegesis (ɛˌpɛksɪˈdʒi:sɪs) (Collins) (plural) -ses (-,si:z) (rhetoric)

1. the addition of a phrase, clause, or sentence to a text to provide further explanation

2. the phrase, clause, or sentence added for this purpose

Example: ' his field or his male servant or his female servant, his ox or his donkey or anything that belongs to your neighbour' is an epexegesis of 'your neighbour's house (beth).'

Epexegetic, and epexegetical, are formed regularly. On the analogy of Athlete from Athletic, the perpetrator of obsessive explanation is an epexegete: (cf. 'exegete' n.)

epexegete (ɛˌpɛksɪˈdʒi:t) (hapaxlegomenon)

(noun) a person who provides minutely detailed explanation.

That doesn't cover the infantilizing attitude this person shows to those around; but it sort of describes the obsession that is so annoying.

infantilizing: treating someone as if that person were a child, with the result that they start behaving like one: (Cambridge online)

Now, a few years later (2021), 'mansplaining'would have to be included in a complete answer to this question. (Merriam Webster)

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:

1. to disturb or agitate suddenly as by surprise or alarm.
2. 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.