For &*&%£$& sake, what is wrong with all you people? Is it something in the water?

1) I would have thought it was pretty clear that the sprained ankle in the OP was supposed to be a "sprained" ankle, ie not really sprained, but claimed (and before you all start laying in to that one, as well, please stop first, breathe, and recall that this is what lots of adults already do).

OP, I wouldn't bother with this option if I were you. The wheelchair service is painfully slow, and there are better solutions. Apart from the moral ambiguity of it all

2) To all of you who have helpfully commented that the child shouldn't be flying on their own if they can't do a transit, I'd just point out that there's a difference in being able to successfully do a point-to-point on your own as a kid, and a same terminal transfer, and a byzantine LHR inter-terminal transfer. Which is why even adults manage to get it wrong from time to time. So maybe everyone could chill just a tiny bit?

OP, she'll be fine, I'm sure. Depending on a) how savvy you think she is, and b) how comfortable she feels about it, I'd suggest one or more of the following options to her:

- follow the purple overhead flight connections T3 signs
- ask a member of BA ground staff on the jetty where to go when she deplanes. They'll either give her some detailed instructions, or if she's one of the last ones off might even offer to walk her thro themerselves.
- ask a member of cabin crew to identify an appropriate passenger sitting near her who is doing the same inter-terminal transit to point her in the right direction as far as the bus/security/departure lounge.

And most importantly, if she ever feels lost, just to ask someone - preferably a member of staff (in uniform) but failing that, even other passengers.

BA wouldn't agree to allocate a member of gate staff to escort her, as you initially suggested, but if there are flight connections helpers at the gate, or if one of the gate staff is feeling helpful and has the time, they might walk her some of the way through...

There, you see - easy!