As someone who also let her daughter be the little nightowl she seems to just naturally be, I'll just toss out that when the time comes that you have to go to bed earlier so that you can get up earlier, you will. It's not worth stressing over.
That said, I don't mean wait until the night before school starts to have your son go to bed 2 hours earlier than his usual conking out on the couch (I love it). We do family bed (I know, I know), so that does allow a bit of control in the when it's time to go to bed department. If you don't want to do the family bed thing, and I can only advocate it for people who truly want it, may I suggest a routine similar to the plan for weaning a child out of it's parents' bed?
You would start by lying down with your child in their bed, moving your bed-time routine, such a bed-time stories, singing songs, telling stories about that day, hopes for dreams, discussing family plans or upcoming events ~ keeping to calm subjects of course. If you don't have a bed-time routine, you might want to create one, especially because you'll need to motivate your son to come off the couch and go into his bed. No judgement - just knowing that incentives help.
OK, I got a bit off-track. So you have the bed-time routine, you're lying down with child. Then you say time for sleep, turn off lights, and stay with them a bit. Maybe, if your child isn't used to going to sleep on their own, you will need to stay until they're actually asleep. This will be quite a huge change for your children, and the less stress the better. Be patient with them (and yourself). The 2 year-old will go along with whatever is going on, allowing for toddler energy and willfulness. But very likely she'll be motivated to do whatever her big brother is doing.
Anyway, you do that for a few days, then sit on the bed rather than lie down, than move to a chair rather than the bed, but close to the bed. Then move the chair further away. As this process goes along, you'll be leaving the room earlier and earlier, so the kids will be going to sleep on their own. I really couldn't say how many days this will take. The idea is to transition them gently so they'll feel safe & not dread bed-time & fight it running away hiding from you. And your night will have some sweetness to it rather than that anxious feeling of OMG my kids HAVE to get to sleep!!! And I do suggest starting the bed-time routine, wherever in the house you choose, a couple weeks before end of summer. You could move to the bed after that, if that feels more natural. The point is to have a routine.
When your kids are little, it's so easy to just float and let life happen. Everyone is happy following their own rhythm in their own time. School changes all that and you will get used to it. I railed against the regimentation, but I got used to it. You have to, or there's all sorts of hassles & stresses that just aren't worth it. Get some stories that you'll enjoy reading (age appropriate), maybe some chapter books that your kids will look forward to hearing the next installment night after night. The ORIGINAL Winnie the Pooh books are terrific, as is the ORIGINAL Mary Poppins, Peter Pan, even Pinnocchio (though there are some pretty tense times in it). They are all excellent books, well-written, funny, sweet.
I wish you all the best with this & I hope you find a way to enjoy the new changes you'll be needing to make. I know it will mean a different night for you the mom, whether you follow my suggestions or whether you go about it another way.