It is illegal to advertise for a nanny of a specific gender. It isn't illegal to choose to employ someone of a specific gender as it is in a domestic household. That's my understanding of the Sex Discrimination Act put very simply.
So an advert, be it on-line or in traditional print media, cannot specifically ask for a male nanny, or specifically for a female nanny. I think you can say something like: Applications from men are especially welcome. But I'm not sure and I'm not sure you really need to do so as either a male nanny will be interested in the job or not, based on the job description, not due to you particularly wanting a bloke.
I'm not sure I would be happy about applying for a job where it was indicated that someone particularly wanted a bloke... feels wrong... feel that someone should hire me for my particular strengths, not due to my gender.
Put things in an advert about the sort of activities you would like your boys to do. If you say that you like your children to be outdoors as much as possible for instance, perhaps that may encourage applications from nannies (both female and male) who like being outdoors, like going for countrywalks, like exploring the neighbourhood and further afield to find places of interest.
I often take the lead from what a child says for an outing idea. If a child says they want to sit in a helicopter, I'll try to find somewhere they can do that (mind you, it won't fly!). If they want to see a castle, we'll find the nearest castle. If they want to have a teddy bears teaparty in the woods, then we find woodland. Any nanny could do that, male or female. By saying you want your children going on outings, may help avoid those people who like staying at home most of the day.