Definitely get differential feed.
If you can afford it, go with a self-threading machine (air threading).
The coverstitch can be really useful (especially for knit clothing), so be sure you understand what it is. Then you can decide whether you want a separate machine for it.
I have a 15-year-old Bernina 4-thread serger with differential feed. If I were in a position to be making children's clothing, I would ***definitely*** have opted for air threading (makes life a lot easier) and either a 5-thread serger (so settings can be changed to coverstitch) or a separate coverstitch machine.
A lot depends on how much you will use it. I would have used mine more if it had been easier to thread and had the coverstitch. Still use it once in awhile (most recently when making replacement leaders for my midarm quilt frame), but not enough nowadays to justify the additional costs involved with air-threading and coverstitch.