Ideal lens/flash combo for a57?

Thanks to some great advice I'm gotten from members of this site, I bought a Sony a57 a couple of months ago. Very happy with the purchase; thanks for the advice!

I have been using the kit lens for a couple of months and now feel it's time to explore beyond the nest.

I mainly take pictures of my toddler - lots of indoor shots - and so would like to have a lens that would be very sharp and quick to focus in low light.

For that reason I was looking at primes. After considering the 50mm 1.4 or 1.8 I had pretty much decided on the 35mm 1.8. This is based on my understanding that since the a57 is a crop sensor camera I won't be gaining much from the 50 1.4 over the 1.8, and that the 35 is easier to work with indoors. If I'm wrong on this, please correct me.

The other issue I'm wondering about is whether I will get frustrated with the fixed focal length. We are going on vacation to Disney in a few months, and so I was wondering if I would be better served to get a Sony 18-250mm f/3.5-6.3 - something versatile so I can spend the trip with just one lens, but still get decent image quality. Would that lens be good enough for the indoor pictures without flash that I would be looking for the rest of the time so I could forego getting a prime? Would it work better/be cheaper for me if I were to get a more versatile (zoom) lens with a flash for using indoors? Would I still be able to achieve some creamy backgrounds that way? What combo would work best?

Also, my idea of image quality is that I could print up to 5x7 (more likely 4x6) with it still looking "good" - just not blurry.

Thanks in advance for your time and help!