I think Yukiko's MAIN concern over Shinichi is that he stays at home way too much reading books and dont care about what's going on in the outside world. So I suppose she was pretty happy when Shinichi actually goes out instead. I dont think Yukiko has to worry over such an intelligent boy. She probably thinks that Shinichi can take care of himself and even if he got into any situation, he would probably be fully capable to get out of it himself. Also, we all know Yukiko's character. She's just takes things very calmly and very innocently. I bet thats why Kudo Yuusaku fell in love with her hahahah. Their characters are just so matching
If anything, I think Yukiko does love her son and perhaps she wants him to grow up and mature fast. It could be that she knew that bad things will happen to him when he got older so she wants him to be able to take of himself at young age.
For Eri's case it is different. I think Eri is very protective of her daughter because she is familiar with all the crimes and evil intentions of people (she is a lawyer). Not only that, it was because of the father's personality too. Since Kogorou is always sooo busy and often neglects Ran, Eri has to be overly concerned with her. Also, in Japanese traditions, girls are considered the weaker of two sexes and families will usually overprotect their daughters rather than their sons. Also, in Japanese tradition, if anything happens, usually the one who suffers most is a girl. That explains the difference in attitude.
Also, another factor could be that Yukiko has a strong trust in Shinichi.
Even if their sons and daughters were to switch, (Ran is now Yukiko's daughter), I believe that Yukiko will still trust Ran (in Shinichi's hands this time round ).