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bakker 110 
Unfortunately you or your girlfriend won't be able to guest into the BA Lounges and a WT to CW airport upgrade cost on the day, if even available, for the children would be prohibitive I think. There's a few choices here I'd say.
1) Pay for all of you to go into the non BA Lounge (Aspire) if you really want lounge access
2) Go to one of the eating places in T5 and chill out there
3) Don't arrive too early if not choosing to pay for the non BA lounge so you don't have to hang around for ages
4) One of you use the BA Lounge whilst the other looks after the kids?
I'm sure there will be a few more posts with other suggestions.