When you say your in-laws will let the dog out at lunchtime, do you literally mean a five minute wee break, or are you talking about a proper walk and some company for the dog?
Because, in all honesty, that's a long time to leave a young dog alone - and labs can be very destructive when they're bored. If your in-laws can pop around at lunchtime and walk the dog (when it's older) or maybe sit with him for a few hours, it could be doable though. Or they could have the dog at their house? My in-laws watch my dog if I have to be out for long periods - but only now he's older mind you 
As for labs in general - they're lovely dogs and generally very trainable - HOWEVER they are also big dogs and very boisterous when they're young. I walk two young labradors (9 months) and my God are they big, strong dogs. They're lovely but they're incredibly energetic and would easily send your 3yo flying if they jumped up in excitement.
How much exercise can you dedicate to the dog on your days off? Can you fit in decent walks with the 3yo at home, for example? Labs need at least an hour off-lead in all weathers in my experience (once fully grown, of course).
I also think you need to think about what you'll do on your working days - there'll be no more impromptu trips to the park after school because you'll need to come home for the dog. No trips to theme parks or the zoo (no dogs allowed) unless you can find someone to dog sit or take him for a walk. Lots of beaches and child-friendly activities are decidedly un-dog friendly too, especially in summer.
When we got our dog, we didn't realise how much of a tie he would be, and we don't have children to think about. We paid for daycare for the first year of his life! I now work with dogs so he can come to work with me - but if I was still in my old job, I'd still be forking out for daycare - which is about £20-25 a day.
I can't leave him for longer than an hour or two - there is absolutely no way on earth he would cope with the set-up you're proposing and he's 2.5 now. Of course some dogs can be left, but I do think you need to go "worst case scenario" - eg. can you afford to pay for daycare or a walker for the next 15 years?