For the younger child write a letter and request an OT evaluation. An assessment is different from an evaluation. I only learned the difference from going through the process at a charter and not using the right words.
This is a link to the process that needs to be followed:
https://osse.dc.gov/publication/rights-parents-students-disabilities-idea-part-b-notice-procedural-safeguards
Write an email to the teacher, who ever you are communicating with and cc the principal.
I wish I could find some examples for you right now - but you want to have it on record ASAP that you are requesting an evaluation. Here is a good link for what to say in the letter.
http://www.parentcenterhub.org/evaluation-2/
The school has a set # of days per DC law to complete the evaluation and determination if they have a learning disability. Given that you are in April - they are going to try and push it off to August when you are back from summer break. Do not let them do this.