I also have an 18 month old son, so I can relate to the challenge of capturing them.
You're going to find it very challenging if not impossible to take many indoor photos without the use of flash. Your child is not going to hold still for 1/15 of a second for you to be able to get away with a lot of available light photography.
So while wide apertures can be great and they do produce some very nice portrait shots, it just isn't a good solution for 90% of real-life shots with a toddler. I'd say my ratio of flash / no flash is 95/5 indoors. The ratio was a bit different when he was a baby and didn't really move. But now, genereatingfast enough shutter speeds in most indoor situations (house, acquarium whatever) to freeze his movement is not realistic.
And the built-in flash on dslrs is still pretty bad. Not very powerful, still get red-eye and recycle times are very slow.
So, yes a 50mm 1.4 or 85mm 1.8 are great portrait lenses. But your overall photography of your child will benefit much more from a good external flash.
So, my suggestion is - get a good external flash (canon 430ex should be fine) and if you want a wide prime for now get a 50mm 1.8 ($70) then save up for the better prime lens.
But life happens at speeds and in light conditions not conducive to slow shutter speeds w/o flash. You can't always position your child by a window. Sometimes there are none and sometimes you want to take pictures in the evening or at night when there is no sun and not much available light in the house or building you're in.
Some examples from a recent vacation - shots that wide aperture, high iso solutions would not have worked well for:
the benefits of 'fill flash':
Birthday parties, christmas parties - etc. All instances where a good flash can have a dramatic improvement on your photos - and all instances where relying on wide apertures and high ISO will yield poor results.
Dont get me wrong - I love wide apertures. And I use high ISO frequently as I do a lot of low light sports shooting. But there are just too many instances where a wide prime and high ISO aren't good enough - especially with toddlers.