Was it really borderline pre-e or was it PIH? These are two different conditions that I have seen used interchangeably, but are in fact two separate issues. Gail Hart has compiled research on reducing the risk of pre-e in women at risk in her book: http://www.midwifeupdates.com/researchupdatesformidwives. I just happened to be looking for something else in there today before I came here. She discusses both dietary changes and supplements that have been shown to reduce risk of both pre-e and PIH. It's a great book, written by a great midwife. Very easy to read and very thorough in its documentation.
In case of you requiring transport, here's what I would do (and have done with a client who required transport for PPH): I would have dh take the baby to the hospital separately from you in his own vehicle. He could wear the baby into the hospital in a sling and make it clear that YOU are the patient, not the baby. (It has been my experience that the hospital personnel will ask to examine the baby, but will not push the issue if refused.) That way you are not separated from the baby for hours, and you are still able to nurse and hold the baby at the hospital as much as your condition allows. As far as the other child goes, it really can be helpful to have a support person who is at the birth or even just available if needed for childcare. BUT, and this is super important, it has to be a person who is supportive of your birth situation, and who does not care about actually witnessing the birth. Do you want your toddler involved in the birth? That is something to prepare for and to consider beyond the "if I have to transfer" question.
Here's something else to consider about inviting ANYONE ELSE to your birth: Having another set of eyes on you during labor can really bring up more issues. I speak from experience. My best friend, who had been at my other two midwife-attended homebirths but is not a homebirther herself, was invited to my UC to care for my 2yo ds. He woke up during a very intense point in my labor and would not be comforted until he saw dh and myself. He calmed down and I'm pretty sure he fell back asleep on our bed as I labored on. But at that point, my bf was on my bed and just watching and worrying (ugh) as I labored. I did not want her in there, but I also could not come out of myself enough to vocalize that to her or to my dh so that he could handle it. That energy of someone worrying and not agreeing with your choices during birth can totally derail things. In my case, it was more of an annoyance and something about my labor that I do regret, but I have seen someone else's bad energy totally throw and wrench in the workings of birth. Just something to think about.