It seems permissible as you are not doing it while making wudu. But if you believe that making your neck wet before or after wudu is preferable then it is b’idah.

In wudhu if I wash my back of neck after completing wudhu or before starting wudhu, so that I’m not making this part of my wudhu. Is it permissible?

Ablution (Wudhu) | July 16, 2017 2:43 am
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I want to ask does washing kids in bathroom break wadhu ? Is there any age limit for this? My daughter is 6 yr old and I wash her after she done with her pee and potty , do I need to re do my wadhu everytime I wash her?