It took six months to Mendoza, Pedro and Sancho to return to Spain. Upon arrival in Barcelona, they were captured by Evil-Masked-Dude and sent to a dungeon, their gold seized.
In the last episode of season 1, when the kids departed the now sunk/destroyed 1st City of Gold, Mendoza and Esteban talked about a rendezvous in a tavern in Barcelona, the very same tavern where they met for the first time. When they (the adults) didn't show up, the kids decided to rescue them.
Evil-masked-dude is also seeking the mysterious cities of gold, he imprisoned Mendoza and co in order to lure he kids so that he could follow them from a distance.