1. Offer to take one or more children for a while during the day. And if you can only take one child, take the more "challenging" one.
2. Bring dinner one night. Or a quick breakfast item. Or mom's favorite snack food (dark chocolate perhaps? or a nice iced mocha?)
3. If mom has a baby, come over and hold the baby - even for 15 minutes. You'd be amazed at what mom can do in 15 minutes of uninterrupted time.
4. Text, email, or call mom. Ask how she is. Encourage her heart. Speak truth to her. Tell her she's a great mom and she's not ruining her children. Words of affirmation go a long way.
5. Fold some laundry, do a few dishes, wipe down a counter - or even just the kitchen table or a kid's high chair. Clean or straighten anything you possibly can. I can't speak for all moms, but this will NOT be offensive.
I might add, asking a mom (of any age kids I would imagine) to "let you know" if they need help won't normally produce a phone call. If you desire to help, it's much more productive to tell a mom what you plan to do and give them a choice of a couple days to choose from. For example "I'm coming to take your girls for a couple of hours. Would Monday or Wednesday be better?" makes it easier for mom to say yes than "Let me know if there's ever a day you want help with the girls". When you want to help, don't ask - insist. Sometimes mommas can be prideful and stubborn and tell themselves they're fine and they don't need help. But we do. We need each other!
I wish our society wasn't so individualistic. And busy. After having had three babies, I wish moms were able to recover and rest longer. I really think it'd help our sanity. :)
Also, I really do love being a mom.