It sounds like your little one not going to nursery until they are 3 is also your decision from a prior response so I’ll try to offer practical advice instead of commenting on your relationship.
With Vinted, every time you upload something it pushes your other listings further up the results too - so try to upload regularly. Take plenty of really good photos (steam or iron any clothes first, take close ups of any wear or marks) and spend a bit of time on your descriptions. As well as clothes, upload any other bits you don’t need (do your older children have any toys they might have outgrown for example?) - buyers shopping that category will then stumble across the clothes you've added too, if they view your other items. Can you sell anything you can collect for free? I’ve seen other sellers listing beachcombed finds (sea glass, shells etc), conkers, pine cones, large collections of cardboard boxes & loo rolls etc for people to buy and use for crafts etc. Get creative!
Keep some of your earnings in your balance and you can use that to buy bigger clothes. You might even be able to buy a couple of cheap presents - set up saved searches and check regularly so you catch any bargains early.
You could also sign up to Prolific Academic - it’s a survey site but the studies are primarily academic research. It pays fairly well, just make sure you’re honest as if you give answers that contradict your profile info your submissions might be rejected. It’s not a reliable or huge source of income but it’ll help. Keep the site open as studies fill up quickly. You’ll need a bit of time and concentration for some (a laptop or desktop computer will be needed for certain studies too) but you could log on while your little one is napping or in bed.
As for Christmas activities and presents, children under the age of 3 want to spend time with & have fun with mummy more than they want expensive gifts. You can set up a lot of sensory activities with things from around the house - try a “snow” shaving foam tray, make a snowman by sticking cotton wool balls to cardboard, sensory rice etc. You can download colouring sheets to print online (the gruffalo’s child is quite Christmassy). You could also bake gingerbread men etc together. Some shopping centres do free grottos - if you can scrimp a bit together, National Trust and RHS sites near us do nice Christmas events / lights for around £10-15 too. Some organisations like this will also offer discounts for those on UC and low incomes.
Hope this helps x