I did Prague, Vienna and Budapest in a 2 week holiday trip over the christmas period 2 years ago. Your allocation of days in both cities look fine to me, personally I feel that Prague has gotten more to offer than Budapest, but you have to be prepared for the amount of tourist you find in Prague! Budapest is a lot less touristy, but I believe it will be popular very soon. You can check out my blog for an idea of how to travel between these countries. http://www.travelathousandmiles.com/eastern-europe/

I've been to both though only to Prague with kids. If you search my name, you will find a trip report for our trip to the Czech Republic that includes lots of kid friendly things to do and places to eat in Prague. My daughter was only 18 months when we went to Prague so if your kids are older, some of the information may not be relevant.

We spent 5 days in Prague and 3 in Budapest. I would keep your allocation as it is. I wouldn't want to spend less than 3 days in Budapest. 4 days in Prague is enough in my opinion.

I'm a much bigger fan of Budapest than Prague owing mainly to the reason given by styrx - the crowds. I couldn't believe how crowded Prague was when I visited (early May and you could barely walk across the Charles Bridge without getting crushed). I hope to revisit Budapest with my kids in a few years. Unfortunately, unless your kids are older (teens), I don't have specific recommendations for that city as the places I went to were definitely more for the older crowd (Museum of Terror, Parliament tour, Opera House tour). If you do want to visit their lovely Parliament, I recommend booking tickets in advance. I posted something about it back in 2008 and you should be able to find that post if you search the archives.

One place I did not visit in Budapest but had hoped to is Memento Park (http://www.mementopark.hu/pages/home/?lang=en). If your kids are old enough, they may get a kick out of the old statues. I went to a similar place in Moscow and loved it.

HK, our family of four (two adults and two children, 11 and 16) are in Prague this week for five days/four nights. A full day and partial morning was spent enjoying the castle complex, even without the Picture Gallery. Exploring the Strahov Monastery and Jewish Quarter took up just about all of a single day, as well. We're also rearranging the itinerary as the weather influences us, so you may want to consider that in your planning.

We took a day trip from Vienna to Budapest this summer, with the specific purpose of touring the castle complex, and wished we had devoted an overnight to see everything. On our way home we toured the Memento Park (not terribly accessible without private transportation) and found it a hoot.

How old are your children? You may find more pointed advice and recommendations by sharing their ages.