Both of these are pretty common concerns for girls of all sizes and ages, trans or not. However, it seems to be sorta hard to find a solution, or answer, aside from having surgery =(
I am definitely one that suffers from having no butt... not only is it not in my genetics to have a big butt, but my genetics also don't allow me to store much fat on my body (which can be key for growing a butt).
Anyway, there is a solution to these problems =) It seems that lots of people grossly underestimate the power of exercising. It is very possible to build up a decent butt and to grow some hip size with just working out your lower body. Since your butt is made up of fat and muscle, it makes sense that building up the muscle in your butt will give you a bigger, rounder, more shapely butt. Just imagine a ballerina, a martial artist, a yoga instructor... all these people have nice butts and hips because of the muscle they've built up in them.
Also, building up the muscles in your thighs, around your butt and in your quads will also help with hip size and curves. Not to mention, the easiest solution to getting the appearance of bigger hips is just to get a smaller waist (cardio and ab exercises)... and a way to diminish broad shoulders for your frame is getting bigger hips and a butt.
Not only will working out your leg, butt and ab muscles give you a more curvy figure, but it will also give you more control over your hips... since moving your hips requires the use and control of your ab muscles.
The ideal solution to the biggest butt possible is to use heavy weights (going to the gym, leg presses, squats, lunges, dead lifts, etc...), but you can also get pretty dramatic results from home too with ankle weights. I bought one of those programs that comes on infomercials at like 2 am (Brazil Butt Lift) to test it out and see if it worked for me. The program is difficult at first, but it totally works. I'm about to finish week 8, and the difference I've noticed is pretty dramatic. There are several different programs like this that are dedicated to building your butt, or you can just look up exercises on YouTube. However, I just liked the idea of a program so I'd get into a routine... and the program I bought focuses on abs and legs too. I'm sorta at a point where I need to buy a gym membership to get more butt growth, but i'm terrified of my parts coming out of my shorts =).... so I will just try to maintain what I have at my house, or either buy some baggy sweat pants for the gym =P
Anyway, here is what I have achieved from sticking by this program. I mean, I'm definitely no 7 of 9 (my friend recently turned me onto Star Trek: Voyager), but I am happy with what I am achieving, and I feel a lot better about myself and my figure.
This pic was at the start of week 8, keep in mind my posture in this pic adds a lot to the shape of my butt

front pic at week 4, sorry for the bad quality

Back pic at week 4