OK, don't send a huge parcel or lots of things to your dad, a board game for the family is perfect and second hand doesn't matter. If you buy from Amazon or ebay you can have it sent to them.
How old are the nieces? I once gave a 12 year old a make over from the body shop, all it cost was my time to book it.
One of the best idea suggestions I have heard for a toddler is to get a box and a bag of balloons. Blow up the balloons and put them in the box, it's really exciting for a toddler to get a box of balloons.
Try charity shops for toys and ask around, lots of people will be happy to help, you need to put your pride in your pocket and ask. You may not want to ask individuals but organisations such as the rotary club, Salvation Army etc often distribute toys to families in need.
Do you have a toy library near by? If your son can understand the concept of borrow then a few toys for over Xmas is a treat.
Food, you are probably thinking, 'how on earth can I afford a turkey and all the trimmings?' Well you can't, but you don't have to. Your children are young enough to be excited by an 'indoor picnic' of sandwiches and cake. Maybe some hot chocolate and a bowl of sweets all on a sheet on the floor.
Lots of animal charities have people fostering animals, depending on your circumstances could you foster a dog or cat? I used to doster for Cats' Protection and I did it all year, but some people just foster for a few weeks a year, often in the summer holidays. Sometimes it was a short foster as the regular oster was on holiday for a week. You would be helping a charity and your children get to have an animal in the house for a week or two. The charity will provide everything for the animal, you just feed and care for it.
If you do want to do a roast and fancy Xmas food go buy it on boxing day and have your meal on the 27th, the children will just be excited to have another special day.
are they loved, safe and warm with full tummies?
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I agree with looking for lower bills for utilities, and also are you aware of the 'warm homes discount' - different power companies have different policies, some catagories of people are automatically selected, others the utility company chose the category. It is £140 credited to your electricity account, if you are on a pre pay meter it is credited to your meter.
www.gov.uk/the-warm-home-discount-scheme