Mi Band 8 Series

I tried the xiaomi after band 8, its so inaccurate, thinks I'm working out just standing still, just ordered the band 10

Xiaomi Smart watches. I have a mi band 8. Love it. There's lots of different options within your budget

I use the Xiaomi Band 8 Pro which is a similar model. I do notice while you swim it undercounts by one length and then corrects itself later so it might a tracking issue. I think the main things are making sure you manually pause each time you stop swimming, and maybe check you've set the pool length correctly? 

The MiBand series are decent. I’m still on the 8pro (your 10 year old won’t need the ‘pro’ models as all they have is GPS) and I had all of them until the 8 pro which is still going strong (for jump rope measuring accuracy). I usually average 150 jumps per minute but it show my max as anywhere from 200-250 per minute, depending on my effort, which is mostly double under sets and seems accurate enough for a base measure. They are relatively cheap (even the pro models) and the battery life leaves all of the other ‘budget’ and even Apple Watches to dust. I have everything on barring always on display and raise to wake and with 2-4 hours training per day it lasts 7 days minimum and anything up to 10 days. It lasted 14+ days but it’s a couple of years old now. I hope this helps.

Di ko alam kung gano ka accurate yung GPS ng Mi Band 9 Pro but I used to have the MB8Pro, OK naman tracking nya, kaso pag walang aid ng phone GPS, hindi accurate yung mapping nya. Tumatalon talon pag tinitingnan mo sa mapa. Never kong binabad sa tubig, pero nababasa naman sya ng ulan and hinuhugasan ko after heavy workouts. Kaso parang may disclaimer ata yung Xiaomi mismo na nadedegrade over time yung waterproofing ng fitness trackers nila.