That the Buddha was a man, and lived a long time ago, are not the most interesting facts.
And even the question 'who was he?' reminds me of 'the unanswered questions' ...
... so suggest you answer slightly different questions, for example: 'what did the Buddha do?'; or 'why is the Buddha important?'; and maybe make it personal like 'why is the Buddha important to us?'
In general, 'near' (in time and space) and 'personal' (in relationships) are more 'important/interesting' than far away and impersonal/abstract.
If you want to describe what the Buddha did, there are verbs you can use: for example the Buddha 'discovered' (or found out), the Buddha 'taught' (or told), ...
If you want to describe the Buddha, consider the Anuradha Sutta in which the Buddha explained how (and how not) to do that. It ends with,
And so, Anuradha — when you can't pin down the Tathagata as a truth or reality even in the present life — [...] Both formerly & now, it is only stress that I describe, and the cessation of stress.
As for how to describe 'nirvana' and 'sangha':
nirvana: Can I presume your daughter knows what 'angry' is? Does someone teach her to recognize/label emotions (e.g. "now, you're feeling hurt/angry because Sarah is playing with your toy", or similar)? Assuming she can label and recognize 'negative' emotions, instead of "nirvana" perhaps you can use negations: "not hurt, not angry, not sad" etc.
dhamma: Here is one summary of the Dharma, as adapted for a UK primary school, i.e.:
The basic tenets of Buddhist teaching are straightforward and practical: nothing is fixed or permanent; actions have consequences; life has difficulties and it is possible to overcome them.
And this suggests it's possible to teach Dharma (i.e. "key principles of Buddhism in a practical way") perhaps without even necessarily saying a lot about the (historical) Buddha himself.
sangha: Would it be reasonable to translate "sangha" as 'friends': e.g. "the Buddha realized [dharma e.g. how to not be unhappy] and then told his friends"? At the famous 'Sermon at Benares' the former acquaintances recognized him as Gautama and addressed him as "friend", and the Buddha told them not to do that (not call him "friend") so I'm not sure about that. I hope it's at least possible to describe the sangha (if not the Buddha) as 'friends'.
Veering off-topic you might also want her to be developing concentration. I asked my mum, who's a Montessori teacher, about 'concentration' in AMI Montessori schools: she said that yes it's important/central, but it (in pre-schoolers) is unforced and comes from the child's interest in the material, that the teacher's role is (to some extent) create an environment with interesting materials and without other distractions / disruptions.