Hi FuntoLearn:
I think Francesco and redskyatnight offer responses which work if you are happy with what the school is doing.
If, however, you aren't happy - say times tables are taught strictly 2, 5, 10 in Y2 and no further, 3, 6, 4 in Y3 and no further and 7, 8, 9 in Y4 (forget about ever learning 11 or 12) - and you personally have an issue with this you may wish to do more.
some useful websites which can be used as redskyatnight suggests to extend learning at home might include:
BBC Learning: www.bbc.co.uk/schools/teachers/ select KS2 tab on orange box on left (middle webpage) and then select area of curriculum.
Coxhoe school: www.schooljotter.com/showpage.php?id=50220 - useful web links for Y4 & their curriculum links www.schooljotter.com/showpage.php?id=35494
Crickweb: select KS2 tab and then scroll down to area of curriculum - here: www.crickweb.co.uk/
Woodlands Primary: primaryhomeworkhelp.co.uk/index.html - also when you click through to a curriculum topic - look out for menus at left or list of zones along the top - these will lead on to other resources.
I agree with redskyatnight strongly that supporting reading, possibly including a regular trip to the library, will bring great rewards. The more they read the better.
If you feel maths is just not being taught in any sort of building block ways and you are aware of real gaps in knowledge you could look into on-line sites such as Mathletics, Maths Whizz or Mathsfactor. I've been involved in some pretty heated debates with teachers here on MN about this - but if you are in a school which is totally unconcerned about learning multiplication (and division just isn't covered) as my DDs are, it is a workable solution for busy parents.
If you are in a catchment for grammar schools you may also want to look into preparation for the eleven plus. There are a number of people who publish work books (banded by age) to use for practice and one or two companies also publish a parents' guide (which is a good starting point) about the 11+. I've started dabbling in these books with DD1 (Y4) and have to admit the fact that she didn't know what plural, verb, noun, pronoun or possessive pronoun was, was kind of startling. In actuality she new roughly, but obviously either was taught another term (i.e. action words for verbs) or they've yet to cover it in detail.
With writing, sneak it in. Have them write in their reading diary about a book (if they don't have regular writing practice). Have them send postcards whenever you go anywhere. Thank you cards for birthday/ Christmas presents, etc... We've found both DDs really can't bear writing for more than 10 minutes, and really do need that regular practice.
I'm not sure 30 minutes or so of work a day is outrageous in Y4 and it is what is recommended (well was, things are changing apparently) for Y5/ Y6 (www.direct.gov.uk/en/Parents/Schoolslearninganddevelopment/SchoolLife/DG_179508). In our case it has meant that DD1, who was below average end KS1, has improved beyond recognition over the last 2 years of doing this kind of supplementary work regularly. I also don't think it's interfered too much with her 'being a kid time'.