100Hz on 32Channels sounds very perfectly doable.
Presuming we are talking 8 to 12 bits, that's only 12 * 32 * 100 = 38400 bits per second of sampling work.
Even 16bit or 32bit per channel wouldn't be a huge drain on sampling budget at only 100Hz, but going above 10 to 12bit with an average photoresistor is probably already more than over-engineering, they are not that consistent between two different types that you need single-milivolt accuracy on your ADC.
If you need exact simultaneous you just need a set of 4, 8 or even 16 channel ADCs, no other option for it. If you need "sort of at the same time" there are many MCUs that offer 200kSample/s to 2MSample/s and 8 to 10 channels muxed. If you have 8 channels on your MCU, you can use an analogue multiplexer of 1 to 4 on each channel and make it 8*4 = 32 channels.
If you need to scan 32 channels at 200kSample/s with some settling time in between, let's say 10us, you will be able to scan them all in:
32*10us + 32*(1/200000)s = 480us.
If you need 100 reports per second that leaves 9.52ms between a sample batch, that's almost a factor 20 more than the time it takes to sample them and it will seem like they are taken at the same time, compared to your measurement rate.
I would advise you some exact MCUs, but since I am very strongly Atmel branded, I feel that would be biased/opinion based. But if you want an Atmel, look into ATMega and ATXMega series, they are easy to use and powerful. The ATXMega types offer a lot of bang for relatively few bucks. But Microchip and Texas Instruments surely have $4 ~ $6 counterparts with cheap experimenter boards based around them that you could use, if you feel there's better examples or support for those.