Hello Bitlost:
First off I've absolutely been there and frankly feel this was pretty normal behaviour in Year 1 for both my girls. Remember in most countries they'd just be in nursery at this age - not starting formal education until the Year they turn 7 - and most of these countries trounce the UK educational on PISA results.
Second: I can only speak from my experience - but it seems to me that at this age they sometimes need to be in the 'I'm doing maths mode' to be able to do this kind of numeracy work - and struggle out of that time/ environment/ approach.
Third - you're still in the first half of the school year and there's tons of time. So relax but also be glad you see there is a problem so early (that is very useful!).
My advice is this:
Familiarise yourself with Year 1 info on new national curriculum Programmes of Study here: www.gov.uk/government/uploads/system/uploads/attachment_data/file/335158/PRIMARY_national_curriculum_-_Mathematics_220714.pdf - list of what is required to be taught (and presumably mastered) starts from page 6.
get the little things right:
shapes: use everyday objects around the house - but basic gemetric shape names should be learned (2D & 3D shapes).
number: able to count to 100 - in increments - by 2s/ 5s/ 10s
if able to do this - consider 3s - use lines of fingers & lines on thumb + thumb nail as hand calculator
calculation: number bonds are important - first work an bons to 10 (and I mean all bonds - all the ways to make 8 is as important as all the ways to make 10) and then to at least 20 (although we found going to 30 really helped with carrying/ borrwing skills).
some really easy things to do at home to help with this:
snakes & ladders - start off counting on how many ever spaces matching your roll of die but then gradually insist that they do the maths in their heads. Maybe first making them mentally calculate 23 + 1. Then similar for rolls of 2. and so on....
add a second die for snakes and ladders to work additions up to 12.
Play the game backwards to work subtractions. you may need to play the board more than once for larger numbers.
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SNAP is a great way to work number bonds.
Use an ordinary deck of cards. Chose a number between 1 - 10 - let's say you chose 7. Decide the range of numbers you'd like to use (let's start with adding numbers 1 - 3 to any number between 1 - 10.
Pull out all 1 - 3 cards from the deck. shuffle them. Write 7 down on a post it or scrap of paper and place next to face down deck of 1-3 cards. Flip first card. Say it's a 3. What is 7 + 3. First to answer 10 keeps the card. Once all the cards have been collected up - the winner is the one with most cards.
You can increase the numbers by making Jack = 10/ Queen = 20/ King = 30.
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Maths with food
I find that the mind is much more concentrated if you work with objects you can eat somehow.
So get in things like smarties, chocolate buttons, raisins, grapes, satsuma segments, baked beans etc....
Make a little pile of beans (for example) - say you have 4. Now ask you child to work out what number of beans you would have if you added 3. Have them move over 3 more beans. They may need to count them up at first - but workign with this kind of visual clue repeatedly helps them learn what to expect.
Subtraction with food is ideal - just eat whatever number you're taking away - so 10 - 7 - just eat 7 baked beans and then count what you have left.
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When you're reading for addition/ subtraction with numbers beyond 10
consider using two related items - so raisins and grapes or lego and duplo blocks.
I find it makes the whole thing easier if you can get your child to understand that the digits in a number represent how many units/ tens/ hundreds/ etc... in that column. So the number 456 is our way of notating that we have 4 hundreds, 5 tens and 6 units.
back to working numbers >10 to about 30....
using to related objects helps you to understand the principle of carrying/ borrowing.
Let's try this with 23 + 8 using grapes for tens and raisins for units.
on one plate lay out 2 grapes and 3 raisins. On another plate lay out 8 raisins.
What do you have if you add them together as is:
count out 2 grapes and 11 raisins.
Well there is a problem there isn't - the units (or raisins) column in a number can only take one digit. We have way too many. So we need to cash some in. How many raisins make a grape? 10 raisins = 1 grape. So we can take 10 of the 11 raisins and trade them in for a grape. (take away the ten raisins and return with one grape). so looking at the plate again we have 3 grapes and 1 raisin = 31. So 23 + 8 = 31.
You can do the same with subtraction
with 23 - 8 you can't take 8 raisins from 3 raisins so you need to cash in a grape to get more raisins. Cashing in one grape from the two for the 2 tends in 23 gives you 1 grape and 13 raisins. You can now take 8 raisins away from 13 (we tended to eat what we were 'taking away') - and that leaves you with 5 raisins and 1 grape which = 15. So 23 - 8 = 15.
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some great free websites:
Woodland Junior School Maths Zone: resources.woodlands-junior.kent.sch.uk/maths/ - especially hundred square/ place value/ addition/ subtraction - links to all sorts of great games - have a bit of a search/ I sometimes find it helps if I try them out first.
Maths Champs: www.mathschamps.co.uk/#home - has games ages 5 - 7. Some can be stressful - so try these out first and decide if it is o.k.
BBC Bitesize KS1 - use easiest level - called 'MEDIUM': www.bbc.co.uk/bitesize/ks1/maths/
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I've posted a lot here about my DD1 and her difficulties at first with maths. My main advice is this: slow and steady wins the race.
Start with trying to do a bit more at home - not overkill - but maybe a quick game of snakes and ladders, playing who can see the most shapes in the living room, drawing shapes, cutting shapes - biscuits?, measuring whilst cooking, etc....
Send positive messages about maths. That numbers are fun, number patterns are interesting, that you're pleased she's improving and most importantly that you totally believe she'll get there in the end!
See how things go before really loading her down with more maths - you do have some time and can try and gently introduce more in Y2 if these simple things don't help.
There are some lovely workbooks: DD2 was given copies out of mythical maths - she loved the pictures and there was reasonably good explanations of how to do things: www.amazon.co.uk/Mythical-Math-5-6-Key-Stage/dp/1844191761. There are a ton of maths workbooks out there - my advice is let your DC help choose - if they like the look of the book they'll be more willing to use it.
If you find that working with your DC leads to quarrels/ pouting/ etc... there are other options - Many have posted (including myself) about various on-line tutorials available and what we've used. I will say this DD1 could barely add or subtract (i.e. couldn't take 1 from 10) in May of Y2 - she went on to achieve NC L1 in maths at KS1 SATs. We started an on-line tutorial - put in between 1 - 2 hours a week (in 15 - 20 minute chunks) on it and she made huge improvements. It wasn't instant, it wasn't easy, it took perseverence and regular practice - but she more than got there in the end.
HTH
It is worth the battle. Genuinely.