I think it depends on where you're doing it. How far do you expect children to be traveling to attend? The football clubs I coached in in the Bronx had spring camps weekly in one case, twice weekly in the other. If you're in a more remote location, then a single day format might be more attractive.
The camps were recruiting tools: no dues, no obligation, no pads -- but we did take attendance just to keep tabs. (Registration was required, for insurance purposes.) I think they were slightly effective at that, but not strikingly so. Both used the same coaches as for football.