It's normal for 12 year olds to be interested in sex. Once they go to senior school they're exposed to the conversations the older girls are having and often asked if they know what a BJ is etc. Of course they don't want to look like 'babies' and not know. Looking things up on the internet, sexual or not, is what kids that generation have been brought up to do. We cannot blame them, or be surprised, when that thing is a term/word/practice that just happens to be sexual.
The 11 yo is still at that age/stage where she'll just ask me, but I don't expect that to last much longer. It's normal to feel embarrassed at asking your mum what x means, much less embarrassing to google it.
The 11 yo is FULLY aware that when she googles something, other things that aren't what she meant might come up. She almost always asks if I think googling something will be ok and often asks me to look first. I'm equally aware this probably won't last too much longer.
Often she will ask why what she wanted to google would mean that there might be things she didn't want to see and I'll tell her (pussy was one).
We cannot stop them accessing things we'd rather they didn't, innocently or intentionally, but we can give them the tools to deal with it.
I'd have words about using her phone when she knows she wouldn't be and I'd tell her that if it happened again the consequence would be x. I'd remind her that it's not a rule I'd made up 'to be mean' but because she needs her sleep, as all kids her age do, but that you can only set the rules for her, not all her friends.
I wouldn't mention what you've seen, just keep an eye on the sites she's spending time on. Better to know than have her hide it if she knows you're going to check.