Oh Gosh! I would be very, very unhappy about this.
The school owes parents an explanation regarding what their plans are for the immediate future and the rest of the year. Given the history, I think parents should be told if this latest teacher is likely to be off for a while, or it is just until the end of the week? As I have said on many threads, Ofsted will see a red mist at this and so should you. Be "that" parent. However, you will probably find there are lots of "those" parents if you look for them! Ask to see the Head. They will wring their hands but this constant change has to stop.
It is extremely difficult to manage classes when there are several maternity leaves and teachers leave. However, the real question is, why are they leaving? (Maternity leave cannot be helped but they do get quite a lot of time to plan for this). Also, why cannot the school get settled staffing in Y1? Is the rest of the school this bad? If it is, my gut reaction would be to leave because it is on a downward spiral.
As a parent, you can do absolutely nothing to help the school. Is there only a single Y1 class? If there is more than one, I would ask whether the school could move another teacher in for part of the week if the current absence is a long one. I also know many schools have supply teachers who are former teachers at the school and know the school and slot invery quickly if they need to. Also, in my experience, when things get this bad, it is possible for a Headteacher to step in and teach. They are, presumably, non-teaching and in a real emergency, with the staffing history in Y1, it would give the children a face they knew and, hopefully, a first class teacher to get everything back on track.
Sadly, with this level of teaching disruption, it will not just be spelling they are not doing - believe me. Who is actually responsible for monitoring progress of all the subjects and making sure all the children are set the work they need to progress? If it is a TA and temp staff, it is likely they will have insufficient skills to ensure all the children make progress. They will be well meaning, but they do not have the insight or knowledge of these children, unless they are very dedicated and skillful. Usually TAs and temp staff cover the lessons that have been prepared by the normal teacher but when it comes to setting the correct level of work, assessing progress and dealing with individual children, they are likely to be less good. You can ask the Head how this aspect of work is being done. I would aslo ask whether the Y1 children are making good progress and ask to see the evidence of your child's progress. I would say very clearly that this level of disruption is not acceptable. I would be worried.