For the argument's sake let's say that one of the kids will not need AHL time and will be ready to step into the NHL full time... that would leave the Flyers with one real banger on D... unless you want to consider Coburn in that category... Grossmann is the Flyers' crease clearer and has his role.
All that said, IMO it is a big assumption that one of the young D will earn a spot on the Roster and be able to play very game against NHL level competition. D-men take longer to develop and often rushing can interfere with their growth... There are a few that can step up, but they are much more the exception than the rule. The Flyers have made a recent strong push for home grown D-men for the future... Homer when GM mentioned that one of them could conceivably force their hands and earn a spot... but IIRC he expected them to need time.
I understand the desire to have one of the kids play with Kimmo, but I feel the timing just didn't fall into place... Maybe bringing Kimmo back as a coach could help going forward. JMO.
I personally don't think you can have both Schenn and Grossmann on the team and be legitimately competitive. They're both the same thing. Slow, handle puck like grenade, lay big hits, block shots. Ones our only Righty and is the same age as our forward core. The other is not. You don't need "bangers" you need guys that can take the puck away and move it up the ice quickly