I teach Pre-K and I too run into that problem. Pictures and circles are a good way to start. A formal assessment based around basic lines, circling, and alphabet are the beginning. Also I would use chart paper during large group so that the students can practice on it, to introduce the concept of your assessment the first 9 weeks, so that by the middle of the 9 weeks they understand what you are asking. I have tried it with Pre-K and they are able to circle the picture and choose what does or does not belong. At least it is a start for those students that do not read or write.

For the formative assessment you can just use a checklist, and for the summative assessment you can do a verbal assessment. Or test whole group and then take students individually who score low to retest.

I teach pre-k and find that true summative assessments such as quizzes or tests are not appropriate. However, using pictures or things they can circle might be more appropriate. You could even ask the questions verbally and write down their answers.