Fresspäckli were an absolute hit in camps I attended. The more sweets they contain, the better
They can be frowned upon because kids sometimes don't want to share, because they can lead to jealousy among the kids etc.
But they can be sent (as TrainDoctor said) to people on military duty with "Feldpost" (for free - albeit only a certain number of packages a day if I remember correctly).
I'm in two minds on the subject - sending your kid a package of his/her favorite candies, foods and maybe a toy/small game will make him/her happy and remind him/her of you, but they sure can spread discord among kids. But in general, Lagerleiter (people who run camps) have rules regarding this issue.