People would be so confused by the Childe x Lumine x Zhongli couple, because to them it seems like Lumine and Zhongli are a couple and Childe is their adopted Childe - Zhongli always fees Childe, because Childe cannot use chopsticks and Lumine is always ginving headpats to Childe like his a good child. So, when Childe brings out the money to pay for everything they are so bewilded - what the hell is going on? Why is the child paying?
But it is just that Lumine is very soft with her boys and Childe loves getting affection and attention, while Zhongli prefers such things to be safed for more private situations - so when no one else is looking, though he won’t push is loved ones away if they insist in showing public affection.
Zhongli has no clue how relationships work or better how to show is love and affection, so he simply mimics Lumine.
Childe does not know how to show is love and affection for Lumine and Zhongli, because both are not very expressive. He keeps his teasing towards Lumine down, because he knows she doesn’t like it. So more teasing for Zhongli - but that dude doesn’t get it most of the time. So he thinks the best way is to buy them whatever they want.
Childe is also the only one interested in sex, but Lumine refuses, while Zhongli already refuses to show “too much” skin, so Childe didn’t even ask. (No matter how cold or warm it is, he always wears a long pyjama and he does not even change in clothes in presents of Lumine and Childe.)
At first their realationship was akward, because no one knew what to do, but by now they are just a happy little family with a lot of cuddling.