Well I think with a Grandad like you she will do just fine, you sound incredibly supportive and switched on, my parents wouldn't even consider helping my daughter with any school work, she's lucky to have you in her corner : )
I'm sure once you can find out the actual methods the school are using to teach maths, with your support your granddaughter will understand and do really well.
I have a similar situation with my daughter with fractions and division at the moment, she's coming home upset that she doesn't understand and is getting it wrong, being kept in at play time to re do the exercises, according to her the teacher doesn't help her though, she just says do it again, you can do it, daughter ends up in tears apparently because she doesn't understand and teacher tells her to stop crying and just hurry up. Obviously this is my 6 year old's version of things so could be way off. I'm going to ask to speak to her teacher about it next week, the thing I want to know is which methods they're using to teach maths so I can then help dd with it at home. Like you we get homework worksheets, but I need to know the actual methods the teacher is using to teach the maths, otherwise my help will just confuse dd even more.
Our school has no info on it's website about this, so I'll ask exactly which methods they use when I can speak to the teacher next week. If you're not able to have a chat with the teacher after school one day because of work etc, does your granddaughter have a contact book for messages to be sent in? Our school uses these so you can write a message to the teacher about various things, if your school uses contact / message books you could ask which methods they're using that way. One thing I'd say though if you don't already know, is that back in Reception the books were all checked daily by the teachers for any messages. Then in year 1 at our school it became the children's responsibility to hand their books to the teacher in the morning if they had a message from home in it. Initially they were asked by the teacher if anyone had any messages to hand in, but then once they got into the habit the reminders stopped and it was totally up to the child to remember if they had a message in the book and to hand it in. So it does rely on the individual child remembering to hand their book in with the message, otherwise in year 2 it doesn't get seen. I'd tell my dd what I'd written, and make sure she knew it was a positive thing that I wanted to learn how she was doing the maths too so that I could help her : )