Check the Premier In - At T4 but easy to reach from T3 by tube and an easy bus outside hotel that drops you at T5. Would take about 45-60 minutes to get there after landing typically. I think you'd have to book the night and add an hour on the other side but it would still be a lot cheaper then most other options. They have family rooms that fit 4 (not 5). There are hotels that offer day rooms but these aren't any cheaper.