just ask them honestly why they dont want you to stay overnight.
and respect thier response. if you go bawling and make a big fuss over it they'l think your too immature.
try agreeing with thier decision, and maybe they'l ket you stay overnight next time, when theyve seen how maturely you took thier decision.
or just try calmly explaining to them that there are adults at the expo, maybe see if theres someone about on the forums who will look after you on the day and maybe be there so your parents could contact them... *im rubbish at explaining things ^^;*
it will suck if you can only go one day. but trust me its better to miss out on one day of expo, than having parents play the whole 'do whatever you want' or 'were not angry were disapointed' card.
Mooooo? OH!! Moooooooo