Quick question to keep me legal!

I co-mind and live with another childminder. We work well together and each have strengths and weaknesses. She hates paperwork/nitty gritty of the job. I therefore sort out all the bits we need - buying contracts/replacement equipment/ink/paper/toys/scrapbooks etc even down to food. I get it ordered and pay for it. She pays me an amount per month - what I ask her for to help me pay bills and all the above. It's not a calculated figure - just roughly worked out so we both have enough to live on. It works for us but is this legal? Would it be accepted by HMRC in the event of an audit? If not how should we be doing it? Any advice welcome!