It will be tough in the beginning but you really need to decide how you want your day to go. Figure out when DD needs to be awake so that you can drop off and pick up DS from school. Then decide when you want your day to start and stop with your DC.
Rough example:
DD wakes up at 7am because DS needs to go to school at 7:30am. Take him to school, come home, make breakfast, get dressed, play for awhile. Have a snack or better yet have an early lunch around 11:30 and have her in her own bed around 12-12:30 for a nap/quiet time.
Get DD up from nap/quiet time around 2pm. Pick DS up.
Come home have a snack with both of them. Play and hang out.
Do your bedtime routine and put DD in her own bed to sleep. If she gets up and tries to go into your bed; carry her back to her bed and tuck her in. It will only take a couple nights at the most for her to learn to sleep in her bed. She will cry and she will have a tantrum but you can't give in otherwise you're perpetuating the problem and making the adjustment harder on her.
Play with the times for your schedule but write it down so you can see it in black and white and know exactly what you intend to do.
Good Luck!