Since my mom passed away, my little dog Morkie is so sad now. What to do?

Dogs generally take their cue from the person they live with and from the here and now. They don't spend a lot of time sitting about remembering the past, tho were Mom to enter the door your little Morkie would celebrate mightily. Your little one is likely picking up on your own anxiety, distress and unhappiness.
If Morkie was used to having someone with her all the time, then the fact that she is being left once in a while would cause stress and anxiety. Look up "separation anxiety in dogs". This can be dealt with easily with training.
You can also consider rehoming. If you find a foster - to -rehome situation that would be best. A shelter would be harmful at this juncture. The foster should be aware of dog behavior and that Morkie is having some separation anxiety issues so they can be addressed. The perfect fit would be a senior chair or bed bound in need of lots of love.
Dogs can transfer their love easily. As someone who took them from shelter, fostered, and found new homes I can assure you that they move place to place easily enough; they just have needs that must be met and they, like humans, form habits. Morkie's habit was to have Mom there at all times.
Best of luck to you. And I am sorry for this loss for both of you.

I think this really boils down to the fact that you can no longer afford to keep your dog than it is about how depressed or anxious she is after your moms passing.
I'm guessing that your mom probably helped with your dogs expenses, and now that she's gone and no longer there to help you, you're having second thoughts as to whether or not you should keep her.
Bottom line, you need to make sure that your money goes to take care of yourself and all that you need before you spend any on your dog. And if you don't have any money left after your needs are met then you know that your dog must be placed with someone that can give it a proper home and will love her like you and your mom did.
It's a tough decision, but you now must do what is not only best for yourself but your dog as well.

Do you still have any clothes or items of your mother's that might have Mom's scent on them?    If you do, place them near your dog's bed, and/or bring them out and let her sniff the items.   I've read that human scents are still detectable by animals, and if she can detect your mother's scent, it might help.

My son's golden doodle was SO nervous he could barely function. He was picking up cues from my daughter in law who's anxiety was over the moon during her pregnancy and then even worse as a new mother. The dog was having seizures, he was so high strung and out of sorts. My son bought him some CBD chewables and after a week of eating them 2x a day, the dog is a puppy again. A brand new animal and no longer anxiety ridden or stressed out, it's amazing. These are CBD chewables FOR DOGS, by the way.

My suggestion is to either provide the proper care required for your dog to thrive, or rehome her. If you can barely afford to keep her as it is, I don't imagine you can afford to try CBD chews for her either, which would likely help a lot with her anxiety. I'm sure she IS responding to the loss of your mother and needs more attention now that she's gone. What lengths are you willing to go to in order to help your dog, that is the question?

Good luck with the dog, and my condolences on the loss of your mom.

I would not place the dog with children since she has never been around them. Maybe find an older woman to take her.

As an animal lover if you can not take care of your dog, or any animal for that matter you need to find another home for it.
Our pets feel loss just as we do. But they are picking up on cues from you. If you are stressed so will your dog.
There are medications that will help with a dogs anxiety just like there are meds for ours. But if you can not afford them and other medications or treatments it would be better to surrender your dog. (I do not say this lightly, older animals are harder for shelters to adopt)
I think you should use a bit of time and "reconnect" with your dog as a single owner. She needs to see you as "her person" again.
Some long walks in the park, around the block will do you both some good finding yourselves. This will ease some anxiety and depression, I bet she is afraid you are going to leave her also.