First question - Do you really want be working until 7pm ?
Second question - How big impact does that have? Not daily but do you have summer time commitment with baseball or Guides or activity for own children at time because they normally before 7pm.
Third question - If have own children, what time they go bed? Lot younger children have 7pm bedtime. How will own children feel if all their family evening time share with client child.
Fourth question - How will you partner feel having extra child during family time? Very different if he come home end of day and it short time before last daycare children leave to having extra child in home all time.
Fifth question - If client child does have impact on own activities, what is the value of that time? You not get that time back, ever. So price to charge might seem expensive for parent but you willing sell your family evenings for $20 a day/$100 a week?

It not about fair price for same number hours other client get, it about fair compensation for losing family time, ability go out for supper, spend valuable time with own children when small, having bedtime routine for own children of supper, bath, story, bed without spare child impacting on bonding, it about having evening time with partner to catch up each other end of day.