I haven’t done foster care but I have had a lot of babies (and the ninth is due in the next month). We always take time off when the baby arrives – the baby IS the lesson. Consider it life skills. Let them learn how to soothe, diaper, and even feed the baby. Let them learn how to do laundry, cook, clean, etc. Focus on a habit you want to see improve in your family.
Now, your 17 year old and 13 year old are probably perfectly able to do some of their work on their own, but let them have at least part of this baby break as well. Then begin giving them assignments.
For the younger two: When you have all had at least a few weeks of baby time begin adding things back in for these two. I would begin with a read aloud – could be history related or not, just some fantastic read aloud. Feel free to use an audio book! Listen to it together a chapter at a time, once a day. Have the 7yo practice reading skills, math skills, and handwriting (copywork). Have the 10yo practice math skills and copywork as well as read 1 chapter independently in a literature book each day.
That’s it. A week later add in poetry and picture study. Literally look at a picture one day. On a different day read one poem by an author.
Keep it slow and build the new routine, all the while giving everyone time to adjust to this new baby and the baby time to grow. (How old is your foster?)
If you really don’t like dropping all history and science for a month or more look for videos or websites the kids can explore. Keep it easy and you’ll be surprised how soon things fall into a routine and you are able to pick back up regular school plans.