Hmmm.
According to Wikitree
Thomas and John emigrated on the 'Earl Grey' in 1841
Mary and her family emigrated on the 'Thetis' in 1848
Hugh and James and their families emigrated on the 'Commodore Perry' in 1856
Andrew and Elizabeth emigrated but no date or ship is mentioned
Jean and Alexander died in Scotland
Isabella is listed but no information is included whether or not she emigrated
Margaret is not listed at all.
In the 1841 census Mary is with her husband Richard Newby and five children in the parish of Twynholm, Kirkcudbrightshire.
Wikitree says that Hugh married Agnes Hamilton and had six children. Scotland's People records the marriage in St Quivox in 1839 and their family as James 1841, Agnes 1844, Jean 1845, Elizabeth Heron and Mary Watson 1847, and William, 1850. (Why did they not name a daughter Catherine after Hugh's mother?)
In 1841 there is a family in St Quivox, Ayr consisting of Hugh Mitchell, 25; Agnes, 25; Elizabeth, 10; Alexander, 5; James, 1 month. Elizabeth and Alexander look very much like the two youngest children attributed on Wikitree to James and Catherine. If Alexander was 5, that puts his birth in 1835 or 1836, so he cannot possibly be a full brother to Hugh, whose mother died in 1833. Therefore Wikitree is wrong about that. The census also suggests that Elizabeth was not born in 1832 as Wikitree suggests, but in 1830/1831.
In 1851 there are in Ayr Hugh Mitchell, 37; Agnes, wife, 36; James, 10; Agnes, 7; Jean, 5; Elizabeth H and Mary W, 3; and a visitor, Hugh Hamilton, who could be Agnes' brother.
James married Jane McGeorge in 1837, and they had Catherine 1838, James 1840, Jane 1843 and Janet 1845.
At Park, Tongland in 1841 are James Mitchell, 25; Jean, 25; Catherine, 2 and James, 1. In 1851 there are James Mitchell, 37; Jane, 36; James, 10; Jane, 8; and Andrew, 2. The passenger list of the 'Commodore Perry' lists James, 40; Jane, 40; James, 15; Jane, 13; Andrew, 7; Catherine, 4; and John, 2.
The gravestone in Tongland Kirkyard confirms that their first Catherine died in 1845 and Janet in 1848, as did another baby, Robert, who does not appear in any other records.