I have said words very similar to this. - ‘Thank you so much for the wonderful slide. It’s a shame our garden is too small to put it up as we had other plans for it. It will have to be at yours. It will be nice for her to have something to play with at your house.’
I think you’re being a tad unreasonable. I’m sure your 2 year old is over the moon! Like the others said maybe you can find somewhere to put it that’s better accommodating?
My mum bought my son a slide when he was 1. We didnt even have a garden lol but he loved it he just used it in the living room and we'd fold it away in the cupboard. Maybe if it really is such a problem, suggest it being a toy that stays at il's house x