This sounds like a very polished classical work. The lines are clear with their own identity and momentum, and the overall form is balanced with nothing lacking. Well done, Brian. I could not do this. Although you may have borrowed from Mozart from time to time, it doesn't really matter because all the pieces fit together as a complete composition.
When you actually modulate, I think it's pretty smooth, actually. However, when you use the borrowed tonic chord, sometimes it isn't as smooth, but not for the reason you think probably. When you go from C major to C minor, I hear it more as you going from I to flat(iii)6, denoting the flat borrowed chord. In other words, I hear it going from C major to E-flat major 6 (or 13, if you'd prefer). I don't know why this is... maybe you have a different bass note...? However, that chord was made awkward further by having the flute play an A-flat in it which is the minor 6 in C and the perfect fourth in E flat, so it doesn't really fit anywhere in totally tonal music. You G major to G minor was better mostly, but sometimes I did hear a flat iii chord instead of a minor i. Other times it was okay. It really depended on where. There was one point (I don't have a time stamp) where there was a B natural and B flat playing at the same time that really didn't sound good. Might want to look into that.
It's definitely harmonically "bearable", It's a little unconventional at times, but I think it mostly works. It functions quite well as a lullaby except for when there are clashing tones.
A small lullaby thing I did for my Music Production (high school) class. The only objective was to produce a piece that will be exactly 2 minutes (my exceeds 120 seconds, but only because of the delay and reverb - I had to leave two empty bars for the sound to go down). I also tried to experiment with the harmony. It's a bit flexible, balancing between C major/minor and G major/minor. I just wanted to get out of the frame of choosing one key and using it's parallel or relative key. On the other hand, I do not want to write atonal music. Nymph's Sleep is written for two harps, glockenspiel, small string section (violoncellos and bassess), flute and bassoon, celesta, and synthesizers (bell synthesizers with a lot of delay). There is no score for this composition yet. Thank you for listening and let me know whether it's harmonically bearable - I did not study harmony yet.
Symphonic Choirs comes with a program called Worldbuilder where, if you the time and patience, you can enter syllables to be sung. I never figured it out but an old member here (not sure if he is still active) who was kind enough to do it for me.