We just were asked to do one of these for a college graduation. Interesting experience and 30 people participated. Some in the montage were better than others. My takeaway - stick to the suggested time guidelines (about 10 seconds to 40 seconds ), stay positive and upbeat , don’t go on and on. A couple of the people just went on way too long (neither parent or sibling did), a couple were unnecessarily negative and mainly focused on the current situation, and a couple were just a little weird. It’s a good idea but remember the recipient will have this to look back on for a very long time. So, do your best!
A couple of other thoughts. The requests to take part as well as the finished product were sent out from the person who compiled this (a parent) and sent to all 30 participants that submitted videos , and all emails were visible. Many of the people I had little connection with and did not need to know their email and would have preferred mine not be shared in a mass email. What you provide will likely end up in a large montage and possibly be shared with lots of people. So keep that in mind as well as how you present yourself.
And be prepared to do more than one take if the first one doesn’t seem quite right. And if very elderly people are asked to participate, help them do their best , especially if a grandchild is involved- coaching, multiple takes if necessary. This is something that lives on and can be very special if done well.