It's difficult, and I really sympathise with you. IME, any lifestyle/lifestage change can be a real test of friendship. I was having a really bad time over a relationship break up when I was at university (silly in hindsight, but I was very cut up about it at the time), and my supposed best friend dropped me like a ton of bricks - she got sick of me being tearful and wanting to talk endlessly about what was happening. Now, of course, I can see things through her eyes - I was being a bit over-dramatic, and wanted someone to endlessly analyse the relationship with. Boring and self-indulgent. But at the time, I did need someone to be a bit supportive and make me feel better about things/myself. Being dropped by a friend at the same time as a boyfriend was incredibly hurtful and now, 30 years on, I still feel hurt/bitter about her and the way she treated me.
You have moved on in your life. Your friends, even if they are sympathetic to your situation, simply don't understand what it's like when you have a child, regardless of the financial aspect.
But I would counsel you to talk to them about how you feel. This is something I never did with my friend. I never made it clear how hurt I was by her behaviour, and I know that I probably became a bit martyrish about the whole thing.
My point is that they may simply not realise how you feel about the situation - they may think that you're totally caught up in motherhood - especially as you are a single parent. Give them a chance to change their behaviour and accommodate you a bit more. And if they don't/won't - then you know for sure that it's time to move on and find friends who are more 'like you'.
One other thing I would say - as someone who is now 'coming out at the other end', so to speak. My DC are 20 and 17 and so no longer require to be babysat. We are now seeing lots of our friends more often who we haven't seen for a while - simply because we all had our own families to deal with. If they are good friendships they will withstand years of being placed 'on the back burner'
Good luck.