Hello dillydally,
We've been to Poland several times - both my parents are Polish, hence my interest in Poland. However, my dh is English, and he absolutely loves it there. The first holiday we took together was in Krakow - neither of us had ever been there before - but we absolutely loved it.
The medieval market square is the largest one in Europe, and it's fantasic in the summer for outdoor eating as it is lined with very good, very cheap restaurants. We have travelled with our son at various ages (18months, 3years and 4.5 years) and he's always enjoyed it, and we could usually find things which were child-suitable in the restaurants, although like anywhere, it's easier to feed adults, especially if you like to eat the local dishes (which we do). The most popular takeaway food is kebabs, and they're fantastic, and cost the equivalent of £1, depending on the exchange rate, and they sell giant pretzels on the street corners for less than 1 zloty.
Last time we went, we rented a gigantic 3 bedroom, 2 bathroom apartment (we went with the in-laws), which came to around £20 per person per night. It was walking distance from the town centre, and directly opposite the fruit and veg market, with a bakery and 24 hour store round the corner.
There's a fantastic water park a bus ride away with a brilliant dragon-themed toddler pool with many water slides, and gigantic tube slides for older kids and adults. You can take a tram to the Jordan Park which has several playground areas appropriate for toddlers, numerous sand pits, a toddler-sized maze and pedalos and bouncy castles in the summer.
I could go on...let me know if you need any more information!