Why are you not watching them when they are being groomed? Even the bath tray is visible from the store side window so you can see what's going on. I always check on my dogs when they are being groomed just to avoid hits, kicks 'attitude adjustments' hair puing and yanking cruelty etc. trust me I've seen just about everything at grooming Salons. So I have dropped in here and there and when my dog suddenly develops a bad attitude, I know they are not being abused at home or at the vet, that leaves the groomer either being a scary place or something else happens there.
I've had a lot of small breeds that need grooming professionally when I cannot. I do bathe mine and brush them out at home, do some hand clipping or use the clippers so they don't go through a trauma every 6 to 8 weeks of being ripped out of the house, being with strange dogs, getting half Drowned, the loud growling air blower screaming at them, usually a flight or fight response for most dogs then they are brushed, mats and all, then clipped.
If your not brushing them out at home and doing a rinse or moisturizing bath between visits the groomer may just be scary and traumatic to them. Smiler dogs usually get hysterical when bring to the groomers unless you have a groomer that is extra careful and takes the patience and time to carefully pull each knot out. Remember your mom running you down for a bath, washing your hair and forcing the comb through no matter how much you struggled. Most two to four year olds go through the I don't want a bath stage, may last weeks to months then one day they ask to go in the tub, total turn around. Most kids I've known do this, I did, my siblings did, so did my parents. It's a kid thing.
Small dogs hate being groomed all the time, with the exception of people that take the time, run the brush through their hair daily so they don't matt up and they keep their sanitary areas trimmed and clean, dogs can learn to growl at and snap at their owners hands that contain the hair pulling weapons.
Next time either stay with the dogs in the pick up area or watch through the store window. If they have blocked off the wash area and windows then try a different groomer and ask if you can stay and watch because of their suspected past abuse. Most groomers will say yes. I always allowed customers to watch as long as they were not calling and talking to their dogs making them behave badly. My clients ( dogs) always went for a walk/run before their bath to get off energy, they had a play area to run in while finishing their drying, they got a treat that the parents left behind, clipped and brushed and out for another walk to relax after so they knew it wasn't all that bad of an experience. My daughter was a part time groomer then she was my dog walker after bathing and blow dirtying the dog.
It may cost ten to twenty more but most private groomers are good, kind and love dogs otherwise they wouldn't be doing that line of work.
Also check at home, keep an eye on family to see if someone gets mad and pops the dogs on their noses or hits them, not saying it's family for sure but if they dive and hide at home I suspect the one day of abuseers may be there besides the groomer. Something may carry on all the time between bathing and grooming sessions.
My minpin had to have his tummy rubbed during his baths or he screamed like he was in severe pain, he had the fight or flight response when all four feet were under water. I filled the sink with baby warm water and lowered him in hanging on to his belly, gently scratching it while the rest of him got shampooed and he loved it. Lower him in holding him by the shoulders, get his but under water then set him down, he started yowling, hysterical barking and screaming until my hand was on his belly. As far as showers alone , it would have been easier trying to catch and hang on to a fly. All dogs are different. My sheltie girl loved baths, she would jump in the tub at home when I bathed my other dogs refusing to get out till the last dog was done, then I would put a no tangle rinse in her hair and let her run outside to dry. She was a nut and loved brushing and bathing so much she would bring me her brush.