L.R.
You're kind to want to help.
You say you "found out" -- Did you find out from her? Or through the grapevine? If you know this for certain is for real, I would just say to her the next time you see her, "I heard that you and X are splitting and I only want to say, I'm sorry to hear it, and I would be glad to help out as needed -- maybe taking the kids somewhere with my kids if you need some time without them for a break or to get things done." And stop right there; you have acknowledged that you know, so don't pursue any questions etc., just the offer of help. I think she would love to hear that. She may say "No, no, it's OK, everything's under control" because she doesn't want to appear needy and needs to appear strong -- not just to others but to herself. So don't push at first.
But in a week or so, if you don't hear from her -- and it is very likely you won't; she is busy and confused and crushed -- get in touch and ask her to coffee, or have her and her kids over to play, or just invite her kids for a play date with yours. Or if you see her at an event, talk to her about her kids, not about herself or the divorce, but you can end with, hey, I'm around to help out if needed. Then make some specific offers so she doesn't say "Oh I'm not sure what help I need, etc."
This is a tough time for her and you may find she seems distant or even downright balky at offers of help, or she may indeed take you up on them right away. Be prepared for either but don't let any refusals on her part drive you away. My very dear friend went through a very sudden split she didn't see coming and it was tough for her to know which end was up or whether it was Tuesday or Friday in the first weeks. So bear with this new friend and stick around even if you don't hear from her for a while. Focusing on your children's common activities gives you a big opportunity just to see her, speak to her every time, and never, ever act pitying -- just open to helping.
You're doing the right thing and you're a good person.