DH and I have been married 20 years and always had a reasonable sex life until the older DC became teenagers and were therefore awake later and would still be awake when we went to bed. House is smallish with walls that you can hear farts through so no question of getting down and dirty if they may still be awake. DC3 also has ADHD and will be awake until the small hours (melatonin did not work on him). Just to complicate matters youngest DC (7) STILL gets into our bed during the night or will be awake at 5am so an early morning fumble is out of the question too.
We had got into a good routine of a nice long session when they were all at school/college as DH normally has one or two days off during the week. This changed when DC1 finished college and is now at Uni so only needs to go out 2 days a week. She won't get a job as she is sooo busy so will spend much of the other days in bed which have clashed with DH's days off. She has now finished until September and won't bugger off out! In addition DC3 has been refusing school since January so he has been at home most days too. Although he has just started going in part time now.
We last had sex in December and I am fed up with it. I have considered telling DC1 that she needs to go out on her Dad's day's off so we can have sex 
but she will obviously be horrified 
and there will no going back from that.
What can we do? There is no spare cash for us to have an afternoon in a hotel. Don't fancy a fumble in the back of the car or an'al fresco' session as wouldn't be able to relax (obvs we can go 'out' and DC1 can babysit) as that would be a waste of time.