3100 vs 5100
D5100 with two lens kit (18-55 VR and 55-200 VR) and then an SB400 is an awesome package for what you want to do. A very capable kit.
You do not have to bounce the flash to get great results. You just need to learn how to adjust flash power so you do not overcook your subject with the flash. Dialing in -1 flash compensation usually does it for closer in shots.
You can also add a little $12 diffuser to the SB400 for indoor shots and that works as well or better than bounced flash. Bounced flash can also give your subject raccoon eyes (bounced light off the ceiling causes shadows under the eyes because the eyebrows shade the eyes).
Note that I have the SB400, as well as three SB600s and two SB800s and very often will use the SB400 for indoor low light work instead of the others. To be sure the others have more capabilites (and I sometimes need those capabilities for my event, sports and portraiture work) but if you learn how to use the SB400 correctly you can get great indoor low light shots. The SB400 is also a lot lighter (less of a burden for me to carry around) and it is smaller an less intimidating to friends and family when shooting familiy gatherings.
The sensor and the CPU in the D5100 is slightly better than the ones in the D3100. That makes the D5100 a better deal and better able to handle more demanding situations. Do not take this too far though since neither have the capability of the D7000. The advantages may be slight, but they are real and over time that will add up to you being able do more with the D5100 than with the D3100.
Later you can have friends or your spouse get a low light prime for you for an anniversary, Christmas or birthday present.