Dog question please help?

hi i have a boxer dog and a cockilier ***** there both 2 and a half yrs old my ***** is in heat n was being kept in the house but my 3 yr old son let her out when i was upstairs i came down and found them stuck 2gether for about 20 mins wot are the chances shes pregnant and what do i do now shes only small and hes a big boxer please help