The Apple Dumpling Gang (1975) on Netflix

Three orphaned siblings are forced upon confirmed bachelor Donovan (Bixby) in a 19th century boom town of Quake City, CA. After an earthquake shakes the area, the children find a large gold nugget worth tens of thousands of dollars. But their newfound wealth is causing more problems than it's solving, so they agree to "give" the gold to two bumbling outlaws (Knotts & Conway). But they only way they can get the gold is to steal it from the bank vault where it's being held for safe keeping.