My brother and I were unschooled and one thing that we benefitted from ALOT at elementary school age was watching PBS documentaries, for example Nova, Connections, etc. We did a lot of projects inspired by what we saw on these shows: drawing things we saw in the shows, writing about them (journaling), explaining them to each other. If you are really wiped out -- and I would be if I were you! -- consider the possibility of watching a few of these documentaries with your kids. They are oddly accessible to small children, and really fun to watch (also, they can be oddly relaxing to watch too... soothing music, etc). I think many of them are available to stream for free on the PBS website. Many are also available on Netflix streaming, if you have that.
If you are not into screen-mediated activities, I would suggest some 'found objects' projects. cardboard, masking tape, and markers can become anything (if your kids are into it). They'll learn a lot from trying to build stuff. Give them challenges (build a playhouse for the younger siblings! build a car that can roll! etc).
Have the older ones teach the younger ones how to do something (anything! make scrambled eggs, count by threes, write their names) and then have a 'show' for mom in which they perform their new skills.
Create a picture book together collaboratively. Create a play about what life would be like if they were pirates/mice/frogs/if the oldest were the youngest and the youngest were the oldest. Perform the play for you.
Or, bonus points for:
Have them create a book about themselves for the baby. Each kid does a page (collage, drawing, writing for those who are old enough, etc). Make the pages individually (on some heavy paper of same size) and then you can bind it afterwards or put the pages in a book with clear sleeves.
The category game. One child names a category and all others name things that belong to this category (this is infinitely adjustable, from 'easy' to 'HARD': animals, things bigger than my head, mammals, presidents, etc etc). When someone cannot name an item in the category that person is out/becomes a judge of others' answers. The last person left gets to pick the next category. You can add a literacy component to this if you feel like it: have the kids who are old enough to write keep a list with the category as a header.
Find a youtube video of kids playing double dutch, challenge your kids to figure out how to do this on their own. You provide rope and some bottles of water.
Alphabet scavenger hunt: kids find something in the house that begins with each letter of the alphabet. If some are too young for this, have them be partners with older kids. You could do a colors scavenger hunt. You could also make a list of things to find (or have the older kids do it) and then challenge them to find the items. Or, for a more literacy based activity, do a book treasure hunt: create a list of things to find (5 words ending in -ing, 6 characters in a book) etc. Have them work together to find them.
hmm, let me think some more...