SuzzyQ

I'm hoping someone here can help me. I'm leading a 4-H sewing group whose members are making pj Pants. Two of the machines don't seem to be threaded properly as the tension either can't be adjusted or seems not to be holding. These machines were donated for our use and I'm used to many different brands but these two have me stumped.
This Singer 774 almost seems like it should be threaded from the left of the tension discs. I think I have the needle position and stitch set properly but the stitches seem wobbly too.
I've attached pictures so you can hopefully see what I mean. The other is a Kenmore and it just seems to get awfully tight - but I didn't get pictures or model number.
We really could use these two machines working properly. We seem to have some kids who really enjoy sewing - this is their first project.