Hi. I am not an expert on London, but we were there for Christmas of 2000 and went with
Halcon Viajes . You should be able to find them in internet. They have cheap flights and lodgings (they own Air Europa, a cheap, but good quality, airline). We stayed in Paddington in a B&B type hotel including breakfast and close to subway, buses, etc. They also provided transport to and from the airport. They had several hotels in that area. Get a public transport pass for your two days and you can take the subway and buses all over the place (the Tower of London is fun). Also, most museums are free and some are interesting for kids. There was also a great natural history museum with dinosaur stuff, but it was not free. We have an almost 6 year-old boy (B-day in September), so I am also always looking for kid-friendly stuff to do, see, etc. Good luck.