Coordinating mulitple travel arrangements

Anyone have any advice on coordinating travel arrangements. My DF and I have decided to move up our moving date and i now need to plan flights.  He is coming to visit here for a week to help me bring some things back with us.  We plan to leave with him when he returns, so we have to arrange a round-trip and a one-way set of tickets that allow us to fly back on the same flight and hopefully be seated together.  I have always used an online site to book tickets, but cant seem to get the flights to coincide the way i want them to.  He will be flying from Manchester to Ohare in Chicago and we'll be returning with him (we=me and my 7 year old daughter) sometime end of February or early March, the sooner the better and the less expensive the better, obviously .