I'm a foster child myself so I know a lot about this stuff. Yes a legal guardian can hold the child 24 hours after their 18th birthday. A legal guardian can hold on to the child for as long as necessary after the age of 18, that is if the child wishes to continue living with them. The only disadvantage of holding onto the child after their 18th birthday is that you will no longer get help or assistance from the organisation that supported your foster child. I would look further into this with the organisation you're with if you have any more doubts or worries. Best of luck :)
A legal guardian cannot hold the child 24 hours after the 18th birthday since this is the legal age that they assume their rights and responsibilities. Parents are usually the legal or lawful guardians of their kids and are always responsible or liable for their care up the the age of 18 years, except if the court orders otherwise.
"Hold the child"? You make it sound more like an in-house detention facility than a home. Upon attaining the age of 18 a person is considered a legal adult and so entitled to make his/her own decisions.