Thanks again for all of your input....
Ted & Kit, my heart goes out to both of you, and your families, as we are fortunate really, that Millie leads a pretty active and normal life....she has occasional days when she has pain, but for most part she is pain-free unless she "overdoes it"...but we have adapted to make sure that she doesn't really get into many situations where she would be likely to do what would be too much for her.
Our problems really are centred around walking long distances - she loves to walk, but she gets to a point and it's like she's hit a brick wall and can't carry on. In day to day life it's not a huge problem - we don't go to the local park as it's a too much of a long walk to the centre where the swings & slides are, but we do go to a small playground a bit further away, as I can park closeby; we don't go out on shopping trips, or if we do, we will just target one or two shops and then head back again; and we don't really go anywhere on foot from home, because the nearest shops and facilities are about two thirds of a mile away and although it's not far for us, it's too far for Millie....but none of those things are a huge inconvenience for us, so we're fine really.
The problems only really arise when we go on outings or holidays, or if there are trips with school - and that it what the wheelchair would be for.
I think you've made a really good point there Kit about being carful that my hubby doesn't give Millie a stigma about what it's like to be disabled. I know he doesn't mean it that way - I think he just worries that we will be viewed like those people who park in the disabled spaces at supermarkets when there is absolutely nothing wrong with them. I think because he despises that kind of behaviour so much, the thought of being tarred with a similar brush hits him pretty hard.
Rosie & Lyndsey - we talked about the buggy very briefly today, and I showed hubby some pics, and he seemed resonably okay with that concept - now whether that was to humour me at the time, whether the whole thing is starting to sink in a bit more, or whether he genuinely found the idea of the buggy easier to accept than a wheelchair, I don't know!
Thanks again to everyone who's contributed!