I also had a lot of trouble with spelling this at first. I've found that it's easier once you know the phonics. Dziewczynka For example, "cz" makes the English /ch/ sound, and "y" makes the /ih/ sound. I'm still not sure about the rules for "z", but it often makes the /zh/ sound, which is kind of like a 'soft' "j" sound, like in "pleasure". Then d+zh 'hardens' it into a j sound.
Oh my, my! All our family arranged a few days in Krakow as a surprise for my husband's birthday. It's only a few weeks away, so I thought that it might be useful to learn a few helpful phrases. Needless to say, I didn't reckon with this! I have learnt several languages in the past, some that were considered difficult, but this one takes the biscuit! It sounds beautiful though. :-)
Polish nouns inflect for 7 cases. Dziewczynka is nominative (base form) and dziewczynką is instrumental:
Ą is a letter that represents a nasal o-sound: https://en.wiktionary.org/wiki/%C4%85#Pronunciation
Well, it's been a while for me, and I don't have kids, so it's not that easy to answer :D But for the alphabet and spelling... I don't think it's different than with other languages. Orthography (so let's say 'spelling') is a problem, when different letters make the same sound (u vs ó, h vs ch, ż vs rz), so there's a lot of practice of those, including a dyktando ('spelling bee') from time to time. OK, it's not a spelling bee as in the US TV, it's just a kind of a test, and it's written.
For declension, I think a first grader generally talks using (mostly) correct grammar, so they just need to learn what the different forms they already use are called. But that's definitely later than in the first grade. I'd guess 4th is the beginning of learning grammar.