Just to update...I heard from yet another person that she is "absolutely furious" about it but to be quite honest, if that is the case, then I think it is for her to express that to me.
Until that happens, my intention is to treat both brothers equally as I always have done and I will be purchasing a second one. To clarify it's a child specific one not an adult one. Their dad has an adult one.
I feel her reported reaction is totally out of proportion to the gift and if she's that concerned then she should get in touch with me rather than venting her anger elsewhere. I don't want to broach it with her because from past experience she is very quick tempered and reacts badly to any sort of criticism.
To the poster who thought I was getting confused about the trust versus my will, it is most definitely not my will and the trust will be administered by myself and another trustee - and hopefully I will be very much alive in doing that!
When the time comes for that, the boys will be over 18 and possibly 21 or beyond depending on how things pan out with them - it's not the kind of money that will be used for frittering away, hopefully it will ensure them a good further education and a foothold in the property ladder. And it's not something that their mother will be consulted about.
I really hope the boys will make more use out of the fitbits than the PlayStation / games on iPads/phones etc (of course properly monitored by their parents as I would expect any such gadgets to be by responsible parents)
Thanks again, really interesting to read all the differing views - I never knew fitbits were quite so divisive. I haven't got one myself but I think I need to try one out.