Access for BF passengers in LHR will exist, though exactly which lounge is unknown. If CO is still in T4 it will either be the SkyTeam lounge or the Holideck. My guess is that CO will not permit guests access but I'm not certain.

In EWR they'd be permitted to bring guests but they'll run into the issue of being underage. If you go in with them it should work out but that is a strange technicality/interpretation of the rule that would be interesting. Since they aren't permitted access as a minor they can't guest you in. But if you were ticketed in BF you could guest them in. Hopefully CO will use logic and permit the access.

On the return you should have the same situation in EWR.