The poop definitely needs cleaning off. If it is left in place and blocks the vent, it will kill your baby very quickly. Also, while there is poop stuck, even if it isn't currently blocking the vent, it will prevent further poop from leaving her vent properly and it will build up and block the vent that way. You can soak her bottom in warm water and gently remove the poop with a Q tip or wash cloth, then make sure she is dried thoroughly, with a hairdryer if necessary, or she will chill.
This problem is called pasty butt and is often caused by the chicks being too warm or too cold. Do your chicks huddle under their heat source and chirp a lot or do they keep well away from it? This would indicate too cold and too warm, respectively. If they happily move in and out of the heat and do no chirp loudly, in a distressed fashion, they are fine.
Food.....ideally they should be being fed on chick crumb. Some finely chopped scrambled or boiled egg will help with the pasty butt and give them a good nutritional boost too. I wouldn't give them flavoured yoghurt....it should be natural, unflavoured, ideally live yoghurt. Apple Cider Vinegar with the mother will help clear that pasty butt right up and keep it from happening again.
I see they are on shavings, so put the chick crumb onto some kitchen towel for now, sprinkle it around and then tap at it with your fingers to imitate a hen showing them how to peck. They'll catch on pretty soon. Also, dip the very end of their beak in the water so they realise what it is. If it is 'open' water, you should fill it full of pebbles so that they can drink from between the pebbles but not fall in and either get soaked or drown. I find that sometimes sprinkling some food into the water helps them to peck at it too but as soon as they have pecked at it and worked out how to drink, you'll need to clean the water out again immediately so as to not leave the crumb in there to spoil.
I realise I have cross posted with
@Birdinhand here but rather than edit, I'll let you take what you want from this info....and welcome to BYC!!! xx