Thats so sad. I would imagine that she would rather know people were there for her, especially as your ds is in her sons class, than think no one cares. If you don't want to knock yet, how about dropping a card through expressing your sympathies etc. and offering practical help such as collecting from school or having her ds round to play. Stress any time in the future as it's often a few weeks down the line that the grief can really kick in.
Take her a nice home made pie or something, so she doesn't have to bother about cooking for one day. And a note with your phone number, offering to look after any/all of the DCs. Apart from the grieving, there is a lot of admin to do when someone dies.
I would make a positive suggestion eg. can I pick X up after school, have a playdate etc (maybe even suggest a date or time).
This is better than a nebulous 'anything I can do to help just ask' although in our recent family bereavement I did ask quite a lot of people to do stuff... most were surprised but happy to actually be asked. I've offered help loads of time but never been taken up on it.
I've also been really crap about phoning people but for some reason can handle e.mail and text messages much better......
Or bake cake. We got none which surprised and disappointed me. We had to buy some!
I'm just helping a family through a similar situation - mum to one of my DD's best friends died very suddenly. Husband has 3 kids (12,14 and 16) and no other family.
My tip would be to be specific in offers of help - I'll come round after school drop off tomorrow and collect some ironing / put on some laundry / take your shopping list with me when I go to the supermarket etc.
Probably a friend or family member will be helping co-ordinate things and offering support - see if you can contact them.
Food is great too - soup, lunches and sweet treats usually also needed.