I vaguely remember that we talked about all this before — citizenship for children and permanent residency with ARI. Here is this awesome post by @susanayang talking about different types of permanent residency and citizenship in length.
It seems that we do have to maintain our investment. Scroll down to “ Option 2 — To Apply for Portuguese Permanent Residence by Investment (Portugal Golden Visa Program)”
In that thread, there was also discussion about citizenship for children after her post.