They don't sound like people you should keep around it's not that hard to find something you all can do
True friends will understand. But as stated before you just have to stick to the same line of that sounds lovely but I’m not able to afford it right now. You just have to be honest. People without children do not realise how much a baby costs. Hell my little one is almost 3 and I’m still learning of new expenses each day. In terms of finances make sure your making the most of what you can get. You may be entitled to universal credit and you can speak to a social prescriber at your GP (if they have that service) who can signpost you to different things you may be entitled to. https://www.entitledto.co.uk
“No” is a full sentence. You don’t need to over explain your financial situation to anyone. It is enough to say you can’t afford it. In the same way they say no to your suggestion to go for coffee you can say, “this is all I can afford” and leave it for them to decide how much they are interested in seeing you. Real friends don’t need more of an explanation and shouldn’t make you choose between having a social life and managing your financial stability in this economy.
I would stick to “I would love to, but I can’t afford it” and then tell them they’re always welcome round yours for a cuppa