Hi. My son is 2 and also bilaterally implanted. I recently asked the same question to a lovely lady I am in touch with who has an 8 year old bilaterally implanted boy. She recomended the helmet that has been great for her and even dropped an old one off for me. 
She uses a met crackerjack helmet but did say that any one with an adjustable back strap (you know the ones with the twisty knob at the back.
i hope this helps x

we have a fantastic helmet which has a sort of elasticated mechanism at the back which makes it so much easier to put on over her implants. you stretch it when you put it over her head, then when you let go, it tightens. 

the brand is Lazer and the mechanism is called Autofit. 
I just googled it and there is an explanation of it: 

Belgian helmet brand Lazer have unveiled a new retention system called Autofit which, as the name suggests, automatically adjusts to fit a rider's head. It then recoils when released, applying pressure and creating a 360-degree fit similar to the manual system

Hi all,

We recently got the help of Chloe & Sophie's Special Ears Fund to find a helmet for our daughter.

It's fantastic. It can expand as she grows older and has special compartments for holding her implants.

We looked around for ages and couldn't find anything, but this really is perfect. You can see an image on their website. (I've added a link above)

Happy pedalling! 

Thnak you guys, I'll have a look at those - thnak you for taking the time to reply!