The donation takes Dorset-based Julia's House's fundraising total to £3.1m of the £3.5m needed to open the hospice.
Chief executive Martin Edwards said it was "wonderful news for Wiltshire" and the pledge meant the hospice could open as early as autumn 2017.
In April, actor Robert Downey Jr helped raise nearly £1m for the project.
Other stars including film director Guy Ritchie, David Beckham and comedian Michael McIntyre have also helped raise money for Julia's House.
Mr Edwards said: "The £3.5m covers the cost of building or converting a property to create a hospice.
"The fundraising appeal doesn't close when the doors open."
The charity has not yet found a site for the new hospice but said it was looking in the Devizes area.
It already offers home-based community care in Dorset and South Wiltshire.
The hospice will allow it to extend its support county-wide to other families with life-limited children.
There is currently no equivalent service for children and families in Wiltshire.
A charity is a step closer to building a new children's hospice in Wiltshire after an anonymous donor pledged £1m to the project.