Anybody know this TV show?

[Donna Halper]

Somebody sent me an e-mail asking about a 1950s-era TV show she 
vaguely recalls from her childhood. She remembers that it was on 
WBZ-TV, "... either before, during or after the Captain Kangaroo 
show.  The few things I remember about this show was it started out 
with a small group of boys (3 -4) that paddle off in a rubber raft 
and they found a cave and entered it and ended up following an 
adventure that just seemed to continue on.  I am assuming that they 
were trying to find there way back to the real world, but each 
adventure brought them further away and they would see dinosaurs and 
large flying birds and such..."

Rings no bells with me at all.  Anybody know what show she is recalling?