Your daughter is just 9. She's training 4 days per week; half-hour dryland, one hour pool. She's a happy swimmer.
"But whenever she's in a meet, her stamina drops clearly at around 35m. As for the freestyle, her 25meter time is around 16 seconds. She usually turns before her friends, falls behind at last 15 meter. It disappoints her."
How can we help her to build up her endurance?
Don't worry about it, don't talk about it, let her and her coach figure it out.
Your job is to get her to practice and provide her with what she needs.
Be happy for her when she does good and support her when she's struggling.
It's hard to say at 9 that swimming will be her sport.
What should we have her eat before the meet?
a small meal but don't worry about it too much, it doesn't really matter.
Should we take her out for jogging, hiking, biking or any other physical activity?
maybe but probably not. She's a happy swimmer, let swimming be her happy place.
Let her drive come from her.
Don't coach her. You may do family physical activities if they are FUN. BUT NO TALKING ABOUT
if you do this it might help you finish your races better.
The truth is, as she gets older and if she loves swimming and progresses, her training will pick up, she'll get in better shape, as she matures she'll get bigger and stronger.
At this point, drive, cheer, shoot video, take pictures, lightly encourage her,
put her in the best convenient program in your area.
let swimming be her thing and I hope it's always fun for her.
Also you're assuming fading in her races happens from lack of conditioning, that is sometimes true, but it usually happens because of incorrect splitting. Trying too hard too soon in a race instead of correct pacing.
Now is the time to have fun & develop beautiful technique which creates a foundation for when she's 11, 12, & beyond.
consider this info:
AN OPEN LETTER TO SWIM PARENTS
here's some swim parent threads