How I tell my step son's I'm dating again

My husband died 4 years ago. My step son's were 11 when my husband passed away. They're twins. He had cancer. They of course went to live with their mom but I was still in their lives. Their mom does not like me one little bit and I don't like her. How she told them they died is she took them out to eat and then dropped on them "Btw your dad's dead." Also used my MILs card to pay for it without her knowledge or consent. When I took them to therapy for her the therapist told me she can't tell me what they told her but just wanted to let me know the boys seem to be being emotionally abused at home. I think the therapist called CPS but to this day their mom thinks I did. While she was under investigation by CPS she told my MIL that either she and my FIL can take the boys or she's going to surrender them to foster care because she's not dealing with this. My in laws are old. They are in their 80's. They can't. So I agreed to take the boys. She signed over guardianship to me and then stopped contacting them. The only time she contacted me was to ask for their birth certificates. She was confused as to why she was no longer getting their social security benefits anymore. They got it from my husband working. She's stupid and didn't understand signing over guardianship means she no longer receives that money. Then tried to say she wanted guardianship back because she was not made clear what signing it over meant. Court didn't side with her. Long story short. LOTS of trauma. Lots and lots of trauma. I've basically been a single mom to them for 4 years. I'm the only mom they have. I just now decided to start dating again and I wanted to know what's a good way to explain that to them. I'm just worried how they will take it.