The kids are sacked out in the back seat.
Simple past tense and past participle of sack out.
The kids sacked out in the back seat before we made it home.
Origin of sacked-out
Outgrowth of the earlier idiom, to hit the sack (“to lie down to sleep”), with possible influences from other senses of to sack (“to tackle, to pillage”), and to sock (“to hit, to slam”), providing an implication that sleep has been thrust upon a person.