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I want to add NFC tag to hearing aid device for a child to monitor on smartphone (or smatwatch - either carried by children) if the tag is in the range - if not it means that child lost it and I can remember GPS position to know where to find it.
Should I expect NFC to interfere with the device?
The range of NFC is a couple of centimetres at best. This does not give enough range to be able to check from a wristwatch if the hearing aid is still present in an ear.
Since NFC works by the active party introducing a strong electromagnetic field to communicate over, it also typically requires a relatively large inductor/antenna, making it unpractical for hearing aids or even small wristwatches. A smaller inductor will work in some cases, but will require more power from the active device. It is quite likely that the strong electric currents would interfere with the hearing aids electronics, coupling into the signal from the microphone.
I would look into Bluetooth Smart, aka Bluetooth low energy for your use case. You could use a low TX power on the hearing aid side to reduce power consumption and many smart watches have support for Bluetooth Smart.
Combining audio and digital signals, esp RF signals always requires careful design to avoid parasitic coupling. The smaller the device the more difficult the task. Start by designing solid power plains and keeping the analog audio signals far away from noisy digital- and RF-signals.
