How long have the two of you been together? If it is a serious relationship, you and he could start to get the kids used to you..and then slowly they would know you were going to be around, were a nice person  etc.. at some point they would not be surprised to see you in the morning - because this disapearing act can't go on forever! Its too hard!

  Another approach is to get the kids to sleep earlier . Kids are plastic-- they may WANT to stay up late but your boyfriend can do some enticements to get them to sleep earlier- stories, maybe a movie or some athletic stuff after school that would have them tired out sooner ( and it would be good for them!)

   He needs to work out a schedule that honors your tie too- perhaps he can get a baby sitter for the kids some nights and come over and see you at your house.. His mother seems to be able to do it some of the time- take advantage of those!

    Work out a schedule together as opposed to doing it night by night. Make sure he knows how hard this is on you and though you want to do it for him and for the relationship and yourself, that it really is difficult. If he cares for you, he will want to either modify their sleeping habits, get help with them or integrate you into the family sooner than later.

Young kids need to be asleep WAY before 11.  I'm not an expert either, I'm just a mom.
That, to me, sounds like an issue all its own.  If the kids act up when they go to the grandparents house, maybe (most probably) the kids are having a hard time adjusting to their living arrangements.  It's tough for mom and dad to split, and then dad have a new girlfriend.  Have you been together for a long time?  I mean, are you new to the kids, or do they know you?
I would first try to help balance the kids schedule.
After the children settle down (I don't mean at night, I mean generally), then you should concentrate on your relationship.  My personal opinion is that the kids come first, and now that dad has them, he needs to focus on them.

As far as sex, could the two of you meet for "lunch"?  Sometimes lunch, when the kids are at school, is the only time of day that we get to be together.  You have to be creative.

I'm not expert, but young kids need a large amount of sleep- I would suggest moving their bed time up-It might be tough, but they need more.  Kids between 5-12 need 10-11 hours of sleep- So if you get them to lay down at 8:30 sleeping by 9:00 ten hours sleep is 7:00 am-

That might give you the time you need.

Also, while they are awake, get them engaged in some activity and sneek off for a quicky-if you have to lock your door so they can't barge in-

If he sets up play dates you need to tell the other parent about the allergy- send your own food over-if it just a few hours they should not need to eat anyway.

sounds like you need to sit down and find the times thats are
regularly predictable, like kids at school or whatever and plan
it then.  not the most romantic way to do it, but you don't have many options.       l.e.

what is your question?

wow, you have your hands full and all your worried about is the sex ?  you need to get some focus, try and work out a new plan for your personal time with the boyfriend. there seems to be an awfull lot of important things going on with his family, where
do you fit in ? whats the big picture here ? is your question
just about when to have sex ?    help us help you     l.e.

Yes, we do have a lot going on.  I fit in nicely with the family, I help with homework, cooking and where ever he needs me.  My question is how do we carve out time for loving making?