This is probably a stupid question, but it is not obvious to me from the instruction booklet. And, I've read postings about other parents' 3 year olds outgrowing their Britax seats, so I am wondering...

I have a 18 month old in a forward facing Roundabout and a 3 year old in the Freeway. Both seats' straps are in the middle position and my daughter often wants to sit in her younger brother's seat and I let her since they are set exactly the same. The booklet's picture for proper strap placement show that for forward facing seats the straps should be slightly below the shoulders and not above them. My daughter is 37 inches tall (30.5 lbs.) and my son is 30.5 (26 lbs.) inches. Am I wrong to have them both in the same setting? Also, I want to put a third seat in my Honda Odyssey minivan (2001) so I can carpool my daughter's 3 year old friends. Can you recommend a decent extra seat? Should I get another Freeway, or is there another better choice?

Many thanks!
Peg

PS. I also use a locking clip with both seats (my seat belts slip through the red built-in locking clip things) and I also use the Mighty Tite rachet tool sold to help car seats be more secure. Are these items helping to keep my children more safe, or do they pose some sort of hazard? Britax says they do not recommend the use of the Mighty Tite, but I can't get the seats immobile without them, so I continue to use them. Are they really safe? Or do they fail in accidents? Please advise!