Do you get any kind of receipt from TS meals when you're on the
DDP? If so, do they itemize what was ordered and/or which type of credits were used for the meal? It would be helpful to have a paper trail of some kind for these kinds of situations.
Either way, I'd at least keep the receipt handy from the Planet Hollywood meal where you used the voucher (assuming you'd get a receipt for using a voucher, if not for using the DDP). That would be "soft proof," should you feel you need it, that there was at least one meal where 1 adult TS credit had not been used.
Just be sure you tell the server upfront about your plans to pay OOP for the kids. If you're a babbler (like me!), you can tell the server all about how you and DH shared one adult meal and used a voucher for another, so you now have 2 adult credits left but no child credits. That should get it all out into the open, get a feel for any possible "suspicion" on the part of the server (at which point you can dig out whatever proof you have in the form of receipts), and also prepare the server to enter your orders into the system the proper way right from the start without having to split the two payment methods after handing you the check.
Of course, if you're really worried, it sounds like the "safest" (least suspicious-looking) thing to do would be to share and pay OOP for everyone (aside from the voucher) at Planet Hollywood. Probably not the most cost effective solution, however. Good luck!