Xiaomi - Smart Band 8 Pro

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.

Coros seems to be the overall pick for best balance between low price and good features. If you're willing to go super basic, the Garmin Forerunner 55 is usually on sale somewhere for way less than its MSRP. You can also get off-brand fitness trackers (usually Chinese brands with a proprietary app that may or may not sync with other training apps) for dirt cheap on Amazon. Amazfit and Xaiomi have fitness bands for less than €50. I used the Mi Smart Band 8 for a while and it was fine. Not great, but it did okay and for the price, it was great.

I have Mi Band 8 and Samsung Ultra Replica attached to P7A

Mi Band 8. F'in great watch and only $35 or so edit: Mi Band 9 has been out a while, but no reason for me to upgrade

I wear a fitness band on one hand and a normal watch on the other. Xiaomi band 8, it's actually amazing. 2 week battery life, tracks notifications and all the basic stuff id even want a smart watch to do for me, and it's small enough that it just looks like a regular bracelet when I put the screen in the palm side of my wrist.

Did you consider looking into "fitness trackers" as a cheaper alternative to "smart watches"? Those are much cheaper and may have sufficient features for you. For example, I got myself a Xiamomi Smart Band 8 because I wanted a watch with a heart rate monitor - it does all I wanted and suprisingly much more stuff that I did not know I would appreciate for just around 30 EUR. Potential downsides: \- cheap as they are, they usually do not have dedicated buttons, so you'd need to use the touch screen to start/pause/stop the stopwatch (which may be less convenient and less precise) \- usually no built-in GPS (but it seems you don't need that) \- most of them are relatively small, so if you looking for something with "bulky" looks, you might not find it in that segment

Xiaomi "pro" band series is a little bit bigger than just a "band", they are rectangle and still smaller than smartwatch, but as it is xiaomi... Well, mine is lagging after less then a year of usage... And never buy "active" series, those are totally useless and stupid