Part-time childcare 10-13 April in Central London

We are a family with three children aged 9, 7 and 3 living in Greece and we will be in London over the Easter holidays. I will also be attending a seminar from Monday 10 until Thursday 13 April so I am looking for a childcarer who will spend six hours per day with my husband and our three children. The routine for these four days will be to meet at 10h00 at the hotel where we will be staying, visit various sites in Central London, such as the SeaLife Aquarium, the Natural History Museum, the Science Museum and the London Transport Museum, have lunch at 13h00 and return to the hotel at 16h00. The children speak English so Greek is not a requirement. Pay for 4 days times 6 hours: 250 pounds.