In a wealthy first world nation I’d say yes, a mattress for a kid is a necessity.
Being a wealthy first world nation is irrelevant "fucking hell" or not. A child or an adult can live without a mattress or a comfortable base. They cannot live without food, water, warmth or toilet as necessities, i would even add clothes on to that.
You can get a childs mattress for £25 from IKEA or £17 from Amazon. Less than 50p a week spread out over the year. If we are a wealthy first world nation then giving a child what you call a necessity is akin to less than a chocolate bar a week.
Are things really that bad for some people in this country that they cannot afford less than 50p a week on a so called item that is a necessity?
Is internet access a necessity? No. Should each child and adult in this country have access to it? Yes. Similar to a mattress in my "fucking hell" opinion.