I spent a lot of time working with poor kids here in the US, both for free and for pay. The short answer to your question is you don't worry about it too much because it comes with the territory.
The other reason you don't worry about it is that the parents are unlikely to sue. Many of the kids who I worked with had parents that were uninvolved, sometimes due to impairment, sometimes health (mental and physical), sometimes incarceration, sometimes death. Other parents are involved to the extent they can be, but are struggling because of divorce, loss of job, health problems with kids or grandparents, fire, flood, any of the numerous events that can make one poor. These parents always have too much to do and too little time - being poor is a job and a half. Basically, a lot of the poor are appreciative for what you do, and too overwhelmed and powerless to do much about it anyway. (You should avoid the rich if you don't want to be sued - they have the resources to sue you, usually well paid, lined up and at-arms.)
My advice past the above (i.e., live with it and hope for the best... always easy advice to give) is too look for a program funded by NGO or GOV entity that can use your expertise. Then limit yourself to that expertise - that is, you don't want to be the Scoutmaster; you want to help kids qualify for an astronomy or science badge.
As for unreliability - you might want to recruit your own team, maybe several teachers you can work with and who you know will show up, or a couple of more volunteers who will show up, or , you know, someone who will show up. Social services are much easier to provide if you have good, dependable people working with you. Kinda like everything else.
Hope you stick with it and have very good luck
Not to mention clear sky