I would sit down with your 6 year old and talk about what a good routine for her would be. Let her help you come up with ideas and suggestions. If you involve her in the process, she's more likely to go along with it, I'd imagine.
I wouldn't approach it as setting up boundaries for her so you can have alone time (though there's nothing wrong with that being the actual impetus here, of course!). I would approach it with you wanting some suggestions and needing some Mama's Helper help.
A silly story maybe, but something that's a strong memory of the way my father once handled something, and it might help so I'm sharing:
When I was young, my grandfather gave me a pocket knife. My mother freaked (unbeknownst to me) but my father stepped in and sat down with me. He asked me if I knew what a weapon was. I said yes. He asked me how old someone should be before they could own a weapon. He wasn't making fun, nor scaring me. He was talking to me like an equal. I thought about it and said "twelve" (no clue why that age popped up in my head but there you have it). Dad nodded then pulled out the pocket knife. He said that he agreed with me and we would put it in his box for him to store for me until I was twelve.
The moral of the story is that he asked for my advice basically, and because of the way he approached the whole thing, I was able to think it through and accept the solution as fair. (He didn't give it back to me at 12 though, but by then I didn't really want it. He would've had I asked though, or probably had another conversation about weapons.
So maybe if you talk to your daughter about a bedtime routine for her in that manner -- asking her input and genuinely hearing and incorporating what she suggests, then I'd bet money she'll be fine with it.
ETA: Maybe even set up a time limit for trying it -- like one week or something and after the week sit down with her and talk about what's working and what isn't.
Then once you've got a routine established for her, look to the routine with your toddler. You might point out during the conversation with your daughter that the routine needs to include bathtime or whatever for the baby too.
Anyway, hope that helps.
And good luck, Mama!