Real scientifical explanetion: ch is /x/, a velar fricative. You put the tongue where you would put it for k or g, but not touching, so there is a turbulent airstream that goes over the toungue. https://en.m.wikipedia.org/wiki/Voiceless_velar_fricative
There are four main conjugations (depending on how you slice it). This page looks like a decent overview: http://www.lsa.umich.edu/slavic/dept/webbasedlanguage/polish/grammar/VerbConjugation/index.htm
When I was in high school, and took German, the teacher I had explained that in a lot of other languages, verbs can be translated three different ways into English. In the case of "the girls have bread", it would be 1. The girls have bread; 2. The girls are having bread; or 3. The girls do have bread. And we just have to figure out from the context which is is.
I'm assuming Polish is similar, but I might be wrong.
Short answer: "to have" is an exception to the rule.
The English "to have" can either mean (among other things) "to possess" or "to partake" (e.g. "we're having lunch," or "he had a good time.")
Although English verbs normally take the present continuous ("to be ...-ing") tense for current actions, the verb "to have" is an a bit of an exception. It uses only the simple present tense to indicate posession (e.g. "the girls have bread"), while the present continuous indicates one of the other meanings.
So, "the girls are having bread" means that they are eating the bread. Because the Polish "mieć" does not have this meaning, "the girls are having bread" is indeed incorrect for "dziewczynki mają chleb."
For an English verb that didn't have this particular quirk, you could indeed translate the Polish present tense into either the simple present tense or the present continuous tense in English.
Yes but trochę chleba is a reverse translation. She has some bread is much more natural than she has bread, which to be honest feels as though it is missing an article and as bread is uncountable 'some' fits the bill. I would not translate 'I have some milk' into Polish using troche. It would simply be 'Mam mleko' surely.