Definitely crop tops/vest for top and cycling shorts for under skirts/dresses (my DD wore a strappy vest and cycle shorts at that age). Cotton vest for summer/spring and thermal type vest for winter. She was wearing cycling style shorts (obviously without the padding!!!) underneath dresses and skirts from about age 4 (when she started school) as she would climb trees, make dens, do handstands etc. Her peers would wear the same, irrespective of size. DD is in Yr 8 now and still wears cycle shorts under her skirts/dresses, as do her friends :) She headed off to school this morning in a school summer dress, wearing cycle shorts underneath. Her best friend is shorter and a larger size and will wear similar. Her preference for pants has been the shorty style pants.
For uniforms, I'd buy a couple of sizes up. It would give space for her to grow and be nice and loose/comfy on her in warm weather. Out of school, we'd get skorts for her to wear in the summer :)
NB for swimming, I always wear an all-in-one that has legs that go down to mid thigh (I'm 5ft 3ish and about 50kg in weight, to give you an idea, but DD is taller and proportionally bigger than me), and my DD wanted similar. A lot of her friends also wear similar. When DD was younger, she'd wear a swimsuit with rash top over and/or swim shorts over. Now she's older, Sports Direct do a Slazenger swimsuit that's an all in one and covers up quite nicely. It has a racer back style so no worries about straps slipping down the arms. Also pretty inexpensive and long lasting! They're simple, functional and - again - it's a comfort, dignity and confidence thing, they can lark about freely without worrying.
One of the PP mentioned figuring out how to talk about puberty. That's a good shout to prep yourself for that now, so you can answer questions or talk about it in an age-appropriate way in the future. When DD was year 4/5, I did start talking about puberty/periods (in conjunction with what was being taught at school) so she'd know what to expect. I also went through different options (period pants and cloth pads) and got some in (she chose a couple of each option) so she had stuff in place for when she did start (unlike me when I started periods and completely freaked out!).
I'm a single parent too and I know it isn't easy. Just take each day and week as it comes. Keep asking questions (none of us parents were born with a chip in our brains knowing exactly what to do!!), and just do what is right for you guys. For my DC (19 and 12), I've never focussed on size (ok, both DC have always been slim), but on healthy lifestyle (good/balanced food, good sleep, exercise etc) and so on. Wishing you guys all the best x