I feel like a shitty friend for thinking this way but I feel as if my friends are taking the piss and I don't know how to bring it up with them without causing ill feelings.
For all appearances I'm a SAHM, in actuality I'm a carer for my two disabled DC (ASD) and I have a small business that I run from home but it's fairly ad hoc and I choose my own hours which is how I like it as it fits around everything else.
My friends seem to think this all means I'm available to water their plants, feed their fish, look after their pets when they go away, dogsit while they're working, and provide childcare as and when needed. I don't mind helping my friends and they do reciprocate if I need it but I don't need it as often as they do so the reciprocation isn't equal. As awful as this sounds too, I find other people's children and pets to be hard work. My own are accustomed to my ways and my habits so we all slot together and are settled into a pattern that works for us all but then an outside dog/cat/child who has a totally different set of needs drops in and the whole apple cart turns to shit. It takes ages to get that equilibrium back afterwards. I hope that makes sense, I'm sure other parents of disabled DC and particularly autistic DC will understand what I mean.
They go on how I'm so calm and capable, that nothing fazes me, that they don't know how I do it, etc. and they don't seem to realise how much work goes into all of that or how finely balanced my life is. My DC seem so chilled out because a lot of work on my part goes into making sure their needs are met and that they don't reach a state of disregulation. Having random dogs and children here upsets that routine, they get resentful, and they end up having meltdowns. My friends seem to think reasoning with them will fix this, one of them even offer to have a chat with my DC to help them understand the situation 
I want to respond that not having other people's problems landed on me would fix it overnight. I'm sympathetic to my friends, I know what it's like to have to juggle responsible it is not my problem that they have a dog and want to go on holiday and it's not my problem that the weather is hot and their garden might shrivel and its not my problem that their childminder isn't well but they're making it my problem because "Clock will help me out". No. I won't. I will pour you a large wine and listen to you rant about your bad day, I'll even join in, but I'm not the solution to your problems. I have my own life and my own problems and you're heaping more onto my already full plate.
How do I start saying no without looking like I'm being a dick?