Sarabela,
That is a late teens to early 20s Singer 66-1. It still has the original presser foot bar that uses the back clamp feet.
Other than that it's a great machine, very much like one of mine.
Have your sister or SIL check in the drawers for the attachments. They can be had, but are not as common as the normal side clamp feet.
Clean it, oil it, and put a new belt on it and it'll sew like new. I love those Singer 66 treadles.
Oh, and this is just my opinion, but I think with the proper feet risers on the treadle pedal, the 6 and 9 year old are old enough to learn to sew. My wife learned at 7 years old and was making her own clothes by 8 or 9.
Love that machine
Joe