Can I appoint a minor (underage) to be my executor / guardian?
Executor(s) / Guardian(s) / Trustee(s) must be above 21 at the point of acting.
However, you can still appoint someone underage (< 21) to be your executor / guardian / trustee. If so, you might want to consider adding another adult as a substitute. You can do so by clicking on the (+) button in the respective sections.
This way, in the case where you die before your "main" executor / guardian / trustee reaches 21, your substitute(s) will kick in.
However, if the Will / Trust kicks into effect before the children comes of age, they will not meet the criteria to become one, and the substitutes (if any) will act instead. This is also why (while not mandatory) it's generally good practice to have at least one adult as a substitute just in case.
In the event you live a ripe long life and your "main" executor / guardian / trustee reaches 21, then they will be appointed accordingly.
If you die leaving a Will but every of your appointed Executors (including substitutes) are unable to act be it they are either dead, bankrupt or simply refused to act, it does not mean that your written Will is all for nothing.
Instead of having your appointed Executor apply for a Grant of Probate, the court will allow for the Letters of Administration with the Will annexed to be granted to such person(s) that the court deems the fittest to administer the estate. This could be a beneficiary or creditor.