Seems like there are two separate but inter-related issues here: Whether there is a positive social effect of creating situations where your toddler can build relationships with other toddlers, and how to handle your ILs feelings about how you raise your kids.
I guess I fall very strongly on the side that while daycare/preschool is not a requirement, it isn't altogether positive for the vast vast majority of the time a toddler has in a week to be 99.9 % with one other person or just the immediate family. At some point your child will have to interact with the world in meaningful ways, and wouldn't you want to help your child (shy or outgoing, comfortable or anxious) navigate how they interact and build relationships from as young an age as possible?
So while trips to the zoo or playground are nice, as someone else said that's still mostly just interacting with strangers and not the same as regular, repeated visits with the same child/children. You can be the most awesome AP parent in the entire world... you still can't create situations by yourself where your child meets with social uncertainty, and has to figure out how to work through that and then feels the benefits of working through it to maybe make a friend or bond with another child enough that they look forward to seeing that child or interacting.
I think there are amazing benefits to being able to be a SAHM and provide that safe, rich enviro for your young'un to develop in. But why wouldn't you also want to create opportunities for your young'un to regularly be exposed to the same kid or kids and let both your toddler and you learn who your child is and where you may want to nurture them further or focus some of your parenting energy based on what you see as their strengths and their challenges?
Then there's the piece about whether you should give your LO time away from you, which to me is a different question from whether they get time with other same age kids on a regular basis whether you're there or not. This to me is more personal and I understand how parents differ on it. Me, I have seen 1st hand how both my own LO and those of family and friends developed when with other people away from me. Dd never spent a night away from both me and DH until she was 22 months, and even then it was a forced work situation that I wasn't happy about. I was so worried how she'd do, but I've learned that as long as you feel really good about where she goes (we don't have grandparents near by so we have to rely on paid childcare, but we found a couple great people who dd already knew through daycare) then it's actually valuable to give dd that chance to see how she does and how she functions. Turns out, while she almost never slept through the night at home at that stage, she slept through the night each time she stayed with S. She was around new kids at S's house and she had a great time, we could see that when we picked her up, she almost didn't want to leave. And we saw how it affected how she was with other relatives we know who we'd like to be able to leave her with once in awhile but we were worried she didn't have enough of a relationship with. One aunt who she hadn't bonded with before, because she watched her for a few hours now dd goes to her first before some closer relatives at family gatherings, because she had the chance to bond with her one on one.
I feel like it's good - when it can be done in a way that's controlled and DH and I are comfy with the person adn the place - it's really good for dd to be away from both of us. Obviously at daycare she's away from us all day, but some evenings too I think is good. And I definitely think it's good for us as adults and a couple to have a little time (even if it's just once every few months) to have a night to ourselves.
PPs have covered the whole IL thing and this is long so I'll stop there. But aren't your in-laws opinions just like everyone else's, where you listen to see if there's anything you may want to try or learn from it, but after that you just go on parenting the best way you can and they have to live with that? Personally I think they ahve a point, but just because their your ILs doesn't mean you're supposed to follow their every word by any means.
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