OP, I was your DIL and now my children are a bit older there's definitely things I felt very strongly about when they were tiny, that I'd be much more relaxed about now - and I can see why they might feel you've overstepped.
What you've got to see is that these days as new parents, the advice is completely different and it's all we know. Co sleeping is basically damned by the NHS, the risk of SIDS is drilled in to you each time you see the midwife/ HV. make sure blankets are tucked under arms, feet to the bottom of the bed etc - as a new parent I stuck to all these rigidly because I didn't want to risk SIDS, the same with car seat safety, sterilising bottles, not giving honey etc. and you darent risk not following it.
Routines were also the things that save our sanity as parents and the tiniest deviation can mean the parents suffer, this was the way for us.
When my parents helped, if they delayed a meal or nap, I'd be so distraught because I really struggled when I was on mat leave alone with the baby all day and always planned for naps at home in the cot because I needed that time for myself to decompress. Napping in the pram meant a shorter nap and no downtime for me, which in turn made me much more snappy and then mum guilt.
We don't co sleep because I don't sleep well with them in the bed and if I'm not well rested I struggle to cope with them the next day. One night in our bed genuinely set a precedent and it'd be weeks before they'd sleep in their own bed again.
It will be all these little things, rather than you personally that's upset them. My parents didn't get this and rolled their eyes or got annoyed at me asking them to do things my way as if I was saying they didn't know how to parent. I'd get eye rolls and "well I've raised 3 children I do know what I'm doing". In reality they only know how to raise their own kids in their own times - modern society has changed.
I only ever asked to do things how we wanted because I wanted to protect our routine and our sanity and their safety, and when it got hard to keep telling them - we did actually stop asking them to have the kids for a while as it was just easier than dealing with the "telling off".
Looking back I appreciate everything my parents did and still do, but my life would have been so much easier and my anxiety much less if they'd been fully on board with how we needed them to be looked after as babies. I constantly felt like I was the bad guy for asking mum to do something we knew was normal - current advice or safety.
In this situation I can see why DIL and son were angry but it does genuinely sound like you just did what you thought was best. I would just explain this and ask what you should do next time, but also they need to understand that you won't always know what they'd do, you only have your own experiences to go from and your instinct took over in this case. You haven't had all the same advice etc as they will have and things have changed since you had babies and they need to give some leeway for that. It sounds like you love them and want to be involved and that is the main thing.
Honestly just talk to them and don't take it personally that they were upset about this x