I completely agree that if they hold racist views - even slightly racist views - they shouldn't be allowed to foster the children.
BUT - as EAP says - we don't know that they are racist. If you join a party, you are not signing up to all of its policies. I completely agree that wanting to end the promotion of multiculturalism is racist, but the point is that it's just one of UKIP's policies. When someone joins a party, there is no way of telling which of the party's policies they subscribe to, no way of forcing them to subscribe to any of them, and nor should there be. If you were the sort of person who lived and breathed Britain's withdrawal from the EU, you could easily want to join UKIP in the hope that you could get the policy on multiculturalism changed. In fact, I can imagine there might well be moves afoot to do just that.
As far as I can see, the only condition for joining UKIP is that you're not allowed to have previously been a member of the BNP etc.
"Membership is not available to anyone who is or has previously been a member of the British National Party, National Front, British Freedom Party, British People's Party, English Defence League, Britain First or the UK First Party. Any applications made from people who are or have been members of these organisations will be refused, and any subscriptions collected will be refunded."