Flight: Go to CPH air port and take the S-train or IC-train from the airport to Roskilde. Take the special festival train from Roskilde to the festival
Train: Go to Copenhagen main train station (hovedbanegård/hauptbanhof) take the S-train or IC-train to Roskilde. Take the special festival train from Roskilde to the festival
Where to stay
There is a camp at the festival, but I’m aftaid that you have to be there very very early to get a decent spot (I’m talking days maybe a week before the whole festival begins) and the camp is pretty awful unless you are really really punk... I would rather pull my hair out and eat it, than spend a night at the camp...
There is hotels in Roskilde, and I suggest you book early
There are rather cheap hotels in Copenhagen (aprox. 25 km away from the festival) Hotel Nebo in Istedgade (Copenhagen’s red light district) is kinda hostel-ish. But it is very close to the main train station. I have spend a night there with my oldest daughter when she was 4, and it wasn’t a problem at all.