I have a Canon 5D Mark III and am looking for a lens for product photography，portrait for toddler and regular travel use. There are too many lens options in the market, so I need some direction to choose the right one for me.
From the info you've provided, it sounds like you're after a pretty general purpose lens, so I'd suggest any of the 'standard' full-frame zoom lenses:
- Canon 24-70mm f/2.8L II (wider aperture for narrow depth-of-field)
- Canon 24-70mm f/4L IS (also has basic macro capabilities)
- Canon 24-105mm f/4L IS (longer zoom range)
- Canon 28-135mm f/3.5-5.6 IS (considerably cheaper option)
Hard to answer because we do not know many things about you & your setup. However I would settle down to Tamron 24-70mm f/2.8 because Canon 24-70 f/2.8 II (the main competitor) is much more expensive and doesn't provide Image Stabilizer (IS). I strongly suspect that the snapshots of your toddler requires this as well as f/2.8 :).
Besides that, a 'regular travel use' for someone who (excuse me) doesn't know what lens to get means that you want to shoot in "any conditions" which means, again, that f/2.8 + IS would help a lot. Of course, this is particularly true if you shoot video.
The other lens offerings for general purpose zooms are with narrower aperture (hence poorer low-light performance) except Sigma's 24-70, but that one doesn't have Image Stabilizer.
If you think that you do not want good shutter speeds (to freeze the motion of your toddler in indoors you need f/2.8 - I don't have toddler but I have enough experience with 5D3 and these zooms in such conditions) then you can go with Canon 28-135 (cheaper but not so good optically) or with Canon 24-105 f/4.
For product photography and toddlers, I'd opt for an external flash before a new lens. For kids, it will freeze the motion better. For product photography, you can setup some really cheap light boxes to get nice even lighting. If you're set on buying a lens for those, I'd recommend any of the 24-70'ish f/2.8 lenses or even a nice 50mm f/1.4 prime. But frankly, flash is the way to go there.
As far as travel lenses go - it just depends on what you want to shoot travel wise. Some people want one ultra zoom lenses to cover everything but compromise on quality. Some people take a series of lenses and deal with the 'hassle' of changing lenses to optimize quality. Rather than rehash all that - take a look here.