This isn't an easy question to answer and the reason is because you could easily say the wrong thing and get your neighbors upset at you. If that happened, they might forbid their children from having anything more to do with you and yet they obviously like you and need you.
At the same time, you can't be substitute parents for these children. You can love them and encourage them (and I hope you will) but they also need their parents. Ask God to give you wisdom, and to open the door for you to share your concerns with them. One approach may be to express gratitude for the privilege of having their children spend time at your house, while showing concern about the amount of time they seem to be alone.
But do so gently and not judgmentally; after all, you don't know the whole picture, and you don't know what strains these parents may be facing. The Bible says, "But the wisdom that comes from heaven is first of all pure; then peace-loving, considerate, submissive, full of mercy and good fruit, impartial and sincere" (James 3:17).
Parents sometimes get busy and don't realize they've gotten their priorities wrong, and this may be the case with this family. Pray for them, and ask God to help you share Christ's love with them. When Christ is first in our lives, we begin to see what's really important especially our families.