This riddle really isn't that hard to decipher. On the scale of Inception, where there's massive loops layers paradoxes and meanings behind it all, this really is probably the most classic aspect of the film to deconstruct, it's a simple poem.
The riddle is essentially setting a very loose metaphor for life and commitment (marriage). Essentially what the characters are promising to each other is that they will stick together and continue the journey that is their life together with commitment. They know how they would like the journey to end, how they would like their life together to turn out, but of course life has other plans quite often, so it's esesentially a promise to each other that they can and will brave any twist and turn of the journey as long as they stay together, and that as long as they're together they will always be in their world together essentially. It's a LEAP OF FAITH into whatever answer the world gives to the riddle, that they'll stay direct and strong together.
The film, of course, covers a sequence of events that become incredibly complicated and philosophically complex. In this case, their journey gets lost to the point that they no longer are capable of doing right by their responsibility to the world, their children, and so Cobb at least within the story he tells has no choice but to perform inception on his wife in order to manipulate their journey together to a specific end, all done with the best intentions of getting back to the children.
An Idea can grow like a virus though, and the inception sticks until Mal and Cobb are left with opposing views of what reality is, they no longer can take the journey together because their models of reality are combative.
The issue is, this rebrands leap of faith, so what once was a freedom inducing leap of faith that, as long as they never leave each other's side they'll accept the journey, now becomes pure dysfunction. They are left incapable of living in the same reality and yet, as the riddle tells us, hopelessly committed to being together, as thsi is their joing model of reality.
The film follows Cobb's journey to finally letting Mal go and coming to the understanding that he had his time with Mal and can't stay dedicated to her. This allows him to rebrand leap of faith BACK into the positive, catharsis filled term it once was for him, as he now brands it as a leap of faith away from requiring Mal and Mal's world (her totem) to confirm reality, allowing him to walk away from the top and be there for his kids, much like he once would have done for Mal.
Essentially the riddle is simple in message and yet the all important key to Cobb's journey or Inception, to make his children the model of reality instead of Mal, since he and Mal are hopelessly caught in an opposing loop. When all is said and done, that's all that's changed, Cobb is again willing to let go of control over where the journey takes him and where it stops as long as he's with who matters most to him, he simply has finally transferred that all important value to his children.
Mal dead wife, children are children. So apply that, and you have a massive metaphor for the maturation of Cobb to let go of the past, his regrets and his stubborn vision of life, so that he can instead make that commitment to the future. Apply philosophical and social implications of that as you may.
But the riddle is simple, it's a leap of faith, a riddle where the answer is not needing the answer as long as you have the person by your side whom defines your world, but this is Inception and as I've covered, shit got real after things got fuckkkkked up, to put it all in laymen terms.