At five and three, your kids have no doubt had difficulty understanding why you and their daddy have separated and will be divorced. They do not need to be further confused or frightened by your acting "touchy-feely" with another man in their presence.
I would introduce your boyfriend into their lives gradually, preferably while doing things with them outside your home. If they know you to be a physically affectionate person with the men and women in your life, then some level of physical affection between you and your boyfriend would not be inappropriate in their presence. However, at this point, that affection should be like the affection you show others whom they know.