hi 3boysmom!!
from experience...YES, WATERSHOES are a MUST HAVE!(or flip flops if you will)...but..I will explain.
Watershoes IMHO are probably better in the long run, and will be more comfortable for your boys, but they WILL have to take them off at ALL slides...the reason is for safety.
The same is true for flip flops.
I do however understand the problem with taking off and putting on watershoes...this is what I have had my kids do, maybe this will help yours as well.
With watershoes, when you are finished riding the slide, make sure to put the water shoes into the water, step out of the pool (which is the landing area of all the slides) and slip your foot into the shoe immediatly..the water inside the shoe will help to ease the shoe back on.(as will your wet feet)...dry water shoes are probably the hardest thing in the world to put on outside of a dry wetsuit.
The problem I found with flip flops, is the occasional flip flop blowout...in other words, your walking along, and bang, the flip flop turn into two pieces(the strap seperates from the sole....making them impossible to continue using)
Whichever you decide, it really is better to have either, than have nothing...the ground, even if it is shaded, is BRUTAL on your feet(and I am not a person to get much discomfort from walking around on pavement barefoot..I go all summer without shoes for the most part)but the walkways at BB and TL seem to be made from the raw ends of fiberglass and razor blades
NO..IM JOKING..theyre not THAT bad..but, still, after a good long time, your dealing with some ripped up feet..the ouchy kind, not the bleeding kind.
I hope you have a great time tho, and like I said...try the water trick..works wonders...and if not, get each kid a spare pair of flip flops just to be safe!!!!
ENJOY!!!