Best would be one row with the 2 children in A and B and an adult plus lap infant in C (so to stop the 2 kids from escaping!) Then one adult with the second lap infant elsewhere - BA only has additional oxygen masks every second row so depending on config the second adult may be either on the same row but across the aisle (in D), or one row behind across the aisle, or two rows behind on the same side (and I would book C or D to be able to access toilet/changing room easily).