Hey tozza,
dont ever lop off the bottom as alot of masts are reinforced there for the extension. If you need to cut off anything do it from the tip, technically speaking may affect the flex minimally, better than the base splitting though. How much are you talking about cutting?

If you are going to cut any mast it is only viable to cut off the bottom. NEVER cut off the top. You would end up with a mast that is super stiff and totally screwed up bend curve. The lack of boom reinforcing will not be a problem for a beginner if you have a good clamp on boom and mast protector, but it will be way too stiff.

For the Severne micro they sell a pretty reasonably priced kids mast that is perfect for them. Another suggestion is to pick up another kids type mast which are often available at very reasonable prices from the right shops.

Other than that, a really soft 370 mast may be ok, like a 370 skinny wave mast or even a 340 rdm but they are pretty rare.

You are definitely on the right track with the Micro sail. It really is important to start new sailors with gear that they don't have to fight with and can have early success with. A wide stable board like a Starboard Start or GO will make a huge difference, as will finding just the right wind and water conditions for that important first experience.

Good luck with it.