What does Mr. Wender ask David to do?

I will assume you are referring to John Wyndham's
The Chrysalids.

In chapter five, after
Sophie's toe has been discovered by Allan, the Wenders decide they must escape. David
asks if he can go with them, but Mr. Wender says no:

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Once they are satisfied that we are out of the
district we'll be someone else's responsibility, and they'll not bother much more about
us. But if Strorm were to lose his boy there'd be hue and cry for miles around, and I
doubt whether we'd have a chance of getting clear. They'd have posses out everywhere
looking for us. We can't afford to increase the risk to Sophie, can
we?'

After it is determined
that they cannot risk David coming with them, Mr. Wender asks David to stay over-night
at the house even after they have escaped.

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'Davie,' he said, man to man, 'I know how fond
you are of Sophie. You've looked after her like a hero, but now there's one more thing
you can do to help her. Will you?'

'Yes,' I told him. 'What
is it, Mr Wender?'

' It's this. When we've gone don't go
home at once. Will you stay here till tomorrow morning? That'll give us more time to get
her safely away. Will you do that?'

'Yes,' I said,
reliably.

Mr. Wender asks
David to do this because he knows David will be implicated in the situation because he
was with Sophie when her toe was discovered. Furthermore, Mr. Wender realizes that when
David returns home, information about their escape will come to light, even though David
doesn't want it to. Mr. Wender wants to delay any information about their escape being
leaked so they have a chance of getting away unharmed. This concern is confirmed at the
end of chapter five as David is crying in his bed after being beaten by his father
saying: "'I couldn't help it, Sophie,' I sobbed, 'I couldn't help it," which informs the
reader he has given information about the Wenders' escape.