Ime it depends who you talk to in SS. We didn't discuss this with our SW on any level. We told them we had a number of issues in the house with DD1's safety and who should we talk to for advice about adaptations? (note at this point we didn't ask for money, we asked for advice, which is cheap. Plus the main thing we were asking for was advice on chewing radiators so a clear and simple no-brainer imminent risk to child's safety.) SW referred us to their OT, and we were lucky to get the head of department who was very good. She wrote a report saying we needed radiator covers ... and a giant cot; we didn't ask for it, she suggested it. It took fecking AGES to get funding agreed, so the thing turned up a year later, but it turned up. And they paid for it without any discussion at all; no contribution from us was ever mentioned though tbh we'd have been willing to pay, it's such a fundamental thing that it's worth it. And there was no discussion/complaint from SW about the locks issue. They didn't even blink.
My suspicion is that in our LA, the OT has a separate budget and so the arguments about restraint/locks when it comes to SLD cases rarely come up because the SW doesn't need to find a reason to refuse funding - it's not their funding so they don't give a toss.
Another thought - I think it helped that I wasn't talking about DD1's safety in terms of stopping her from leaving the room, or stopping her from chewing the walls or whatever. It wasn't about stopping her doing something she wanted to do. The issue was that she has to have a grobag rather than a duvet because she lacks the processing power to work out that she is unhappy because she is cold, and getting under the duvet will get her warm again. So she'd never get a night's sleep without the grobag, she'd just stay up shivering and crying. But even on a mattress on the floor she'd be unsafe in a grobag because she'd wander about at night and trip and bash her head. So the "for her own safety" thing was more about stopping her doing dangerous things by accident, rather than stopping her doing things she wants to do that happen to be dangerous.
It's absolute bolloxs, but I get the impression that's key; ie when it's something the child wants to do that Social Care gets all silly. If you physically stop a child from doing what they want, they call it restraint. If you stop them from doing what they want by sleeping across their doorway and waking up ten times a night shouting at them to stop it, well that's parenting. Utter nonsense of course, either way the child can't do what they want to, and probably they'd find the predictably locked door easier to handle than the unpredictably shouty mad sleep-deprived harried woman - but that's the logic of the parallel universe that is Social Care. 