The weight used in the CE calculation isn’t the weight at the time of the ride according to this post Category Enforcement Update - Weight Changes [December 2022] it’s an average of weights around the time.
If your kid is 15 or under there is no excuse you should just get a free account for them. If they are over you should really pay for an account or they will have to start riding with a weight penalty. It is really good of Zwift (or evil and it is to get them hooked) to offer free accounts for kids, I’m going to have to start paying for my kid later this year 
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