Hi Longdiling:
I think you've got the right idea - if math's difficult - doing more can help and learning through game play seems less like 'formal learning' (e.g. worksheets/ practice papers/ etc...).
Some great free websites with lots of resources:
Woodlands Junior Maths Zone: resources.woodlands-junior.kent.sch.uk/maths/ - just select the topic and explore the games. I used to try them out first and then suggest DD1 had a go.
BBC Bitesize KS2 Maths: www.bbc.co.uk/bitesize/ks2/maths/ - I found this useful to determine where gaps/ problems were. Then I could target practice in those areas (we did this at KS1 when we realised DD1 was seriously behind - but I think the same principle works at KS2).
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If you feel there's just a huge amount that needs doing you could consider an on-line tutorial. These are by subscription and prices vary - but many (including me) have posted here about success with various tutorials:
Maths Whizz www.whizz.com/
Mathletics www.mathletics.co.uk/
Komodo Maths komodomath.com/
Mathsfactor www.themathsfactor.com/
They all have different formats & styles but I think all offer sample lessons/ free trials so you can have a look (and include your DD) to determine which suits.
For us, we went with mathsfactor, because at core DD1 needed someone to explain all over again how to perform certain mathematical functions and to remind her what she was doing. She liked the video lessons (hosted by Carol Vorderman) because if she didn't understand she could just rewind.
I don't know what the other programmes offer - but all will provide an opportunity at further practice to really bed down concepts & improve speed - and I think that really helps build confidence.
From our perspective doing a bit more at home (just 1-2 hours spread out over the week) really seemed to make a huge difference.
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maths is something like a foreign language and sometimes kids just miss that important concept or need a bit more time to get used to doing things a certain way. I also think sometimes schools are rigid about what method to use, even if it isn't the obvious or natural way a child might tackle the problem, and this certainly was at the core of my own DD1's issues with maths in Y3/ Y4.
The new national curriculum (I know this is for England but am unsure if this applies for Wales) outlines quite clearly what should be mastered by year and can be something of a guide about what gaps/ extra work needs to be done to get back on track. www.gov.uk/government/uploads/system/uploads/attachment_data/file/335158/PRIMARY_national_curriculum_-_Mathematics_220714.pdf
Hang in there and keep working at it. It may not seem like it's making a difference but gradually it will. I know from my own experience with a child who was seriously struggling the really important thing to signal is that you totally believe they will 'get it' in the end. Your DD is very lucky to have your support. 
PSBD