@Letsgobacktothenineties
You need to stop asking your parents. It's your home, a 2 bedroom, you have a DD and at 3.5 years old she needs her own bedroom. There is no liner a spare bedroom.
The fact your Dparents have enjoyed a double bed and spare bedroom available at yours to Stay for this long is a luxury. It's not a "contract" forever more.
60s-70s isn't old either , may parents are 80s.
OP you find the solution you want. Don't ask your parents , tell them what you decided suits you. Be that a sofa bed or a trundle bed (2 in one pull out) , or a blow up double mattress (you can buy £50 luxury high ones) but if you like the little girls ikea bed you bought for your DD stick to it. Don't sell it. That's her bed and bedroom 24/7. Do not build her life and choices around other people who may or may occasionally want to stay at your house s( 1/10th the year) to visit !! It is not their home, it's DDs home, you're, DDs bedroom, her bed, her life. Your parents have their own bed they have chosen at their home, why doesn't your DD get the same?
Your parents have limited choices when they want to come visit. You offer your bedroom (- meh dog or not , it is what is on offer tell mum to stop moaning / being ungrateful) , sofa bed in lounge or blow up bed? They can try either of those. If they don't like it after trying it or don't want to try them, they are free to do what other peoples parents do- which is book an air b&b or a bed &breakfast or cheap hotel for the 2 weeks they want to visit you for. That they pay for. It's not for you to pay for their holiday stays to visit you. If they moan, then tough, that's the available options. You're not a child, you are a parent yourself who has to put DD first
Your DM sounds as if she's become entitled and is treating your DD as an object, without feelings, that can be moved around to suit her as your mother. Not many loving grandma should do that. My mum (grandma) wouldn't dream of this- she'd be the first one saying "let's get you a bed little one (DD) and help decorate your room so you have your own space"
. And my parents in their 70s slept on blowup mattress in lounge at same age when we had smaller house (having been offered my bed)