I like the pendent idea for the girl with a picture of the two of you and an ingrave of I'll always love you. Or maybe a best friend pendent you know the ones that break in 1/2 and each of you wears one. You could have the backs of theirs engraved with yours and bills initials and add some verbal sentimental words of how much you love them,Bill is now apart of that love that's why his initals are also on the back and that they are your best friends as well as your children and end it with the always love you bit. Or you could buy rings similar to yours and Bills and say we are all marrying eachother b/c we are all family and these rings symbolize the sharing of that bond and in the ring have all of your initials carved into each one for your son and daughter. Yours,Bills,son,and daughter all around the ring.
Not original I know but that's all I have Unique. Let me know what you decide on.