Moving in Together

My boyfriend of almost a year and I are thinking about moving in together. He has a daughter (6), and we are struggling to decide how to split bills. I suggested he pay for himself and his daughter, with a 60/40 split on rent/utilities. I suggested this because I know she is still little and he wouldn’t have to pay quite 2/3 of things but it’s not split down the middle either. He wants everything to be 50/50, as he claims she is only 6 and how much could she really be adding onto bills. I explained that because she would be living with us 50% of the time, the added bedroom to accommodate her adds to expenses alone. Not including water bill, etc. He has no problem paying for the groceries she needs and any other thing that is not household bills, and that has never been an issue between us. I only spend money on her when I want to. Is there another way I can explain this to him? Where should I go from here?