The family (mum, dad, daughter 10, son 8) saw this at the cinema and really enjoyed it. Its works well on the small screen as well and has quite a serious storyline, although to be honest the kids have only watched it a few times, so perhaps try renting as well - they prefered to watch their many hours of TV Wild Thornberry episodes on video instead, as there is far more variety (and even I enjoy watching them). However this is a very successful movie translation with a strong conservationist story. It's not unlike Tarzan really with it's England public school sequences. Your children would probably appreciate the film more if they are familiar with the TV series (but then they will have probably seen it already). Unfortunately the TV series isn't being screened at the moment. I think the film will still make perfect sense if you aren't familiar with The Wild Thornberrys - it's just the exact reason why Eliza can talk to animals that is glossed over, although to be honest that only took up a few seconds of the original 90 episode TV series anyway. Eliza's constant kindness to animals was rewarded by a powerful African witch-doctor/shaman spirit who granted her the ability to talk to animals (not that she can change their inate behaviour much though) - however it's on condition that she must keep this gift completely secret to prevent it being misused by other humans.
I far preferred this movie to the later 'Rugrat's go wild'. It's particularly of interest to a UK audience as Nigel Thornberry is very English, and a cross between David Attenborough and George Sanders (Sheer Khan in The Jungle Book). Plus any film with Brit Tim Curry involved is always fun (the voice of Nigel). There's also Lynn Redgrave as Cordelia Thornberry - Nigel's mum. The film particularly suites girls as both main leads are girls, although the animal action sequences and wild-boys Donny and Darwin keep the young men happy. Highly recommended for a 5 to 11 audience (and their parents), particularly as now it's an old title, it's quite cheap. It's also actually available to buy cheaply in the UK, unlike sadly most of the classy TV series - although a few single PAL Wild Thornberry DVDs, e.g. 'Heroes of the animal kingdom', 'Call of the wild' [6 episodes], and 'The origins of Donnie' [4 episodes], are available to rent and buy on Amazon at the moment. The Wild Thornberrys TV show hasn't actually been cancelled either, it's technically been put on 'hold' for a few years as there were so many episodes already made.