To me, microscopes are like pacemakers for your heart, they only come in two main categories, junk and not junk. The good news is pacemakers are way less complicated.
In short, 400x (on a $100 or so compound microscope) SHOULD certainly get you all the quality high school level bio class stuff a pond has to offer, without a lot technical skill to try to see it. BUT it is about CLARITY not just zoom. For pond watching, you want a compound (or phase) microscope with four main ingredients - a light condenser, a 10x objective piece, a 40x objective piece, and a 10x eyepiece. ?Why:
Most of the time, unless you are going to spend considerably more money on something that claims really high magnification (ie 1000x - 3000x etc) you wont get clarity and detail, which is everything.
to get best results for a beginner - First you have to look at what is called the "objective piece" (which is the nozzle looking piece that actually looks down into the specimen - it is a lens mechanism) your microscope MUST INCLUDE one objective piece with a 10x AND an object piece at 40x (but NOT so much a 100x - 100x is not needed and requires more work, 60x is good, but NOT 100x unless you are already expert level). These objective pieces should also have a minimum - lets say 'quality' - rating of "achromatic 160" or otherwise an "infinity" rating on what is called the "DIN rating" on that "objective piece." Decent quality microscopes usually have this rating printed right on the objective piece and objective pieces are removable and swappable. Usually, your good basic microscope has 3 objective pieces with different magnifications ranging 4x - 100x. The eyepiece (where you actually look into) should have at least a 10x magnification. The two of these together (40x X 10x), will give you an enlargement of 400x which is the benchmark for "good or quality." This shopping list also gives you the best value for your money. In fact, you can spend 100 bucks or less for these essentials, while others will cost you 100 bucks or more and DO NOT offer these essentials even though they come in brighter, prettier colors. You can also spend $300 and get a microscope with these min standards that will last her until college (and is upgradeable). SECOND - assuming she is not doing anything like virus research yet - the compound or phase microscope should have a light condenser (which would be a red flag if it did not).
The only other thing I think is important is to make sure it is a plug in - not just battery operated. That tends to go hand in hand with better quality, but doesn't necessarily mean more expensive.
The things that I mentioned are all the minimums that are inherent in microscopes that give a quality CLEAR image from most, if not all, of what you want to look at from a pond (but you can go much deeper). This can be had for $100 in the year 2020 or 2021. However, these very same microscopes that have these standards will typically include many more useful features (same price - all included).