I always thought if a child was in the Magnet program, siblings were a shoe in. That really stinks! I do know that for pupil placement (which is what would be your option here) is only available if you can prove a family/childcare hardship. I think your chances would be better if the times were not as far apart. You will at least get the bus to HWES now, if you did not get to ride when you were there only as a magnet student since bussing is provided for AAP eligible students.
Generally speaking, if the school is closed for Transfer, it will be very difficult. Looking at the Cluster 8 schools (of which Crossfield and Hunters Woods are both a part of) it looks like Hunters Woods was closed for transfers.
http://www.fcps.edu/fts/dashboard/11-12dashboard.html
My thought is for you to go to the principal at HWES to explain the situation. Most families I knew who had older children in HWES in the magnet program had siblings join with little issue, but this was some years back.
I had my children at 3 schools this year (dd at Reston Preschool, ds in K at Dogwood, ds in 4th at Crossfield in LLIV) and though it was far from convenient, we survived since there were different start times. Another option if you truly do not want to split them is to do Local Level IV at Crossfield.