Hi Nick, I think you have some great images here.
Companions: I love the moment you've caught here... great timing and as a father myself (also of a little boy and baby girl) I can empathise about how these sorts of image communicate with you so clearly. The main thing to say is it's your photo, so it should please you and your close family... don't worry about the rest of us! However, if you're looking for some independent thoughts (as someone who has struggled to get the right shots of the wee ones), I'd make the following constructive (hopefully!) crit:
1. the lighting you've used is definitely of the "moody" variety... this is great and it's certainly more interesting than the more typical very soft lighting used with babies and younger kids. However, it doesn't necessarily accentuate the best features of said babies/kids and is also why it's used rarely in such circumstances. Youngsters (babies especially) are generally considered to be "bonnier" when they are fuller featured (within reason) - big cheeks, obviously those huge eyes and chunky little arms. As an example, we're a skinny family but our youngest (1.5 years) still has loads of baby fat and even her older brother looks fuller in the face than we do (these are good things at that age btw!). Therefore, lighting which accentuates the roundness of features tends to be more pleasing - this is big soft lighting which doesn't draw too much attention to cheekbones, jawlines etc. by softening the shadow areas at these "normal" shadow locations.
2. Polly's necklace looks interesting so I'd quite like to see more of it - this could be achieved with a shadow lift but these things are always better done in camera with a variation on the lighting if possible.
3. There are quite strong shadows under Archies hand and also being cast on him by Polly - nature of the lighting which would be softened if you used a larger light source. Again, this is a matter of choice and you've said you chose your lighting deliberately.
4. Finally, babies and toddlers move very quickly and don't often like posing for long... you may have noticed this
Therefore, by using a larger, softer light source, you accommodate a much wider range of shooting angles and positions which are still acceptably lit. Harder directional light with kickers etc. is fine when you're shooting a static, posed subject (e.g. an older child or an adult) but your hit rate with FMCLOs (fast-moving child-like objects) will be very very low - frustratingly so and more often than not, the moment you want to capture will be the one which isn't lit right!
5. Window light can be a great friend, especially on a cloudy day. This is because you can shout out to your model: "what's out of the window" and then they get lovely soft light falling on their face just when you want it (sometimes not having eye contact can be more interesting).
I still think you've captured a magical image and if you / your partner likes it that's by far the most important thing. If you want pictures which the rest of the family go "aahh" at even more, then some of the tips above might be helpful. If you're using flash/strobes do use walls and ceilings to bounce, especially with kids as it creates a massive light source. I have a fairly large softbox for my strobe (1.2m) but I actually prefer to bounce the flash when shooting the kids - it also takes less time to set up
Elegance: quick comment here after all that - lovely shot, really well composed and a very very novel take on the theme, which I really like. I'd echo
@Dark Knight 's comments about having just the right amount of guitar visible too - cracking shot!