A couple with a baby and occasionally a 3 yo. (they have partial custody) wants to rent out the other room in a 2 bedroom apartment. We all split the rent and are on the same lease. Water, garbage, trash are in the landlord's name and are accounted for on the lease agreement. Electricity, gas and internet are in my name (current tenant) and are not included in the lease agreement. Can I split the utilities per person living here? Or can I only split them per adult living here? Because it doesn't seem fair to me to have to pay more in utilities on my end, when babies and children use gas and electricity too. Also, would that count as a significant change in the lease if they already moved in and the lease is month to month? Because I believe that may qualify them for a relocation fee if they move according to Oregon law.

I live in Portland, Oregon.