How important is feeder primary in the admissions criteria? It may well be that they'd get into one or both of the secondaries regardless of which primary they're attending. If so, you might not need to worry about secondary at this point, or one primary might leave the choice for secondary and the other not.
Size of school affects how easy it is to avoid your child. I worked in a large school where it was very easy for staff to avoid teaching their own child. I even know of one parents evening appointment where the teacher was completely unaware that the parent was a colleague! It can also depend on subject - if you're the only music teacher, you'd have to teach them, and if child is an active musician it could be difficult. If you teach maths, it's probably always going to be possible for you to teach a different set (although one of my colleagues asked to teach her son's (top) set on the grounds that then she was less likely to be disciplining kids in his form who might take it out on him).
You might move job before your child reaches 11, in which case this isn't worth worrying about now! If not, and your school is not near home, you need to consider whether your child joining in year 7 would then tie you to being there another five years - I know one person who was itching to apply for something new, but was stuck because her child was at her school and would have to move if she did.
With recruitment the way it is, more schools are adding in a "staff children" criterion to their admissions policy. If your school does that between now and then, then being at one of their feeders may not matter.
It may also depend on your kid's character. It's probably too soon to know that, but by the time you're actually applying for secondary, it should be clearer.
Some schools have more staff kids than others, just due to geography. Where there are more, they're probably more adept at dealing with it. Are there any staff kids currently in your school? Those families would be good to talk to.
Some families operate a system where the non-teacher parent leads on all interaction with the school, so that contact is forced down the formal routes, rather than "having a quick word". That gives some chance that the child feels there's the same distancing between home and school as other kids have - rather than "mum knows every time I'm late with homework".