My opinion only...I feel that you shouldn't tell them that you or they don't need a "dad" around in order for all of you to be happy or to be a family. If you were a single father I would say the same thing. Children benefit from both a mother and father figure in their lives. Boys need both a mother and father to teach them how to be men. Moms teach them tenderness, give them encouragement, show them how to love, hug, and treat other women with respect. A father figure teaches boys how to be boys and men. There is a saying, "it's easier to raise a boy to be a man than to raise a man." Men have a special place in a young boy�s life when it comes to manly activities, sports, etc. Just as your daughter needs a mother to teach her about emotions, boys, men, menstruation, how to be a girl a father figure can speak positive in her life, make her feel like the princess she is, teach her how she should be treated when it comes to boys and then later on men (fathers have an incredible power over their daughters when it comes to a daughters self-esteem and how they allow boys or men to treat them) and a father figure can also make her feel that she is the most special and most beautiful young lady in the world;someone to be revered and honored.
Short but true story. We went on a beach trip for 3 days with friends of ours. I was pregnant and they had a 12 year old daughter. Not that their daughters father isn't wonderful but he had issues at the time. Unknown to my husband and I my friend had told me after the trip that her daughter made a comment that when she was ready to get married she wanted to marry someone like my husband. She had noticed how caring, attentive, gentle and loving he is to me. My husband's eyes swelled with tears when I told him. It touched him to know that he had been such a model for this precious young lady.
Children desire a mom and dad unit, to them it just makes sense to call that a family. To teach them otherwise is teaching them too much independence and they grow into adults feeling that they do not need anyone in there lives. As true as this is in many aspects such as we can't depend on another person to make us happy or to be fulfilled in life we also need to make sure that our children understand that in circumstances where one spouse is not around or within the immediate family unit it's still wonderful to go out and enjoy life, meet others and if the right person comes a long how great it can be to have both a mom and a father figure back in the same house. Not to mention double the love the children will receive.
It is precious that your children want you to find love and happiness. It only natural! Thank your son for his desires in your life as his mom and let him know that he can hope for whatever he wants for his mom and you never know, one day it might happen but that you want to make sure it will be with someone that loves them and wants the best for them as much as you do.