I am a single parent to children aged 10, 5 and just turned 4 and am about to have a newborn, too. Until recently, my youngest napped during the day and so could stay up until the same time as his sisters at night. However, since dropping his nap he could really do with going to bed earlier, as could DD5. DD10 doesn't need much sleep and could easily be up until 10pm.
Ideally, I'd like the two youngest to be asleep for 8.30 at the latest. However, DD10 is awful at being alone and would no way read quietly in her bedroom until 10. She'd expect to be downstairs with me and then would be likely to be loud and wake the others. At the moment she doesn't have any access to technology and hasn't asked for any so I don't really want to encourage it - particularly just before bedtime. How do other people manage bedtimes with different aged DCs?