Personally, I don't really think it's fair to feed other people's cats, even if you are trying to help - which clearly you are. You see, it could be a vicious circle - she doesn't feed them enough because when she gets home they aren't hungry, they aren't hungry because you've fed them, you've fed them because she's not feeding them.. you see my point.
If she's new to having cats, it will take the young ones to be begging for food for her to realise that perhaps just one meal a day isn't enough. Otherwise she'll never learn.
I agree with what others have said - perhaps you could politely let her know that the kittens are coming to you for food, ask her if she feeds them in the morning. Or perhaps you could say something like "when my kitten was that age, they just loved... (food name here) would you like to try it with yours?" - then offer her some. If you start the conversation in a way that's not judging her, she might even ask you for advice - and then you can speak freely about the food and the shed!

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