I think your concern is legitimate and I understand why you're worried for the little boy. If he were old enough to be in nursery or school I would talk with them
I think it's clear you don't think he's in danger. The mum might be unsupported, depressed, maybe she didn't have a positive childhood with all the lovely normal experiences you facilitated as a childminder.
The fear with Social Services or Children's Services or whatever is always that some middle class people with clipboards are going to come and take your child/baby away and make judgements about your life.
In our school, as in many others in the UK, we use a reporting system called My Concern, this is really useful for building a picture around a child and seeing the patterns and ways in which they can best be supported, or where something might be a one off or actually lots of little things add up. I don't really know what's in place for pre-school babies and toddlers.
I lived in Flying Start areas when my children were new and little so I had free access to lots of parenting courses, baby play, talk and sing, baby massage, all sorts. And I know it wasn't aimed at me because I already saw value in nursery rhymes and reading to my babies and chatting to them with full words and sentences and getting down on the floor to fully engage with them. Because that was my norm and that's what I saw from family members and other mums around me and was what I remember as an older sibling of many. Some cultures and communities don't do it so much. Some parents who've not seen it modelled won't do it naturally. Some people don't do loads of research about parenting and some do.
My hope would be that this mum does make the change and engage as she'd get so much more out of the relationship too. Mainly though, the little boy will have a better time of it with speech and language development, social skills and emotional wellbeing.
Is there a way to chat with her and maybe invite them both over to spend a little bit of time at yours or at the park or in your garden? A chance to connect with the mum but also to model (in a very gentle and natural way) talking and playing with the boy, singing nursery rhymes, word games etc. Doesn't have to be anything big or fancy, something like "I spotted these bubble wands on offer at the shop and thought he'd enjoy it" and blow them in the garden while he chases them and talking about bubbles "ooo that's a big bubble, can you make it pop?" "yay, POP!"
I see the comments from people saying mind your own business but I genuinely think your concern has come from a place of care. Of course as mums we all have down time, we have the moments of scrolling or drinking, but it sounds like this family aren't also getting the fun interactive stuff and that's sad. Also, you've come here for advice, you've not chatted in the supermarket tutting about this mum, I believe that you want to help the mum to make a change.
We have so many 3 year olds come to nursery with lower levels of speech and language development (and toileting and so on) than children the same age in the same deprived area a decade ago. What you've described in this household in a huge part of that. She's probably caring well for him in every other way - clean clothes and bedding, good food, tooth brushing etc but that lack of play and communication really does matter.
I think it's a two pronged thing: 1. you building a relationship with the neighbour so there's someone she can trust and that can model positive communication, play and cheap days out (park, beach, museum) with the boy and 2. give the NSPCC a call for some specific advice around who you could talk to.
This advice from Action for Children could be helpful and suggests giving the NSPCC a call: Get help if you're worried about someone else’s child