Camping in August: Sleeping bags?

Hi everyone, We'll be in Namibia, in the usual locations, for the last 2 weeks of August. Our 4x4 rooftents come with duvets, pillows and sheets but we're conscious that the temperatures will drop a lot overnight. We're a family of 4, with boys aged 12 and 15: do people think we should bring sleeping bags with us from the UK, or is that overkill? On the one hand, we don't want to spoil the trip shivering in the night but on the other hand we don't want to pay for extra luggage to accomodate 4 bulky sleeping bags! Thoughts?