IMPROVIA® Washable Underpads, 34" x 36" (Pack of 4) - Heavy Absorbency Reusable Bedwetting Incontinence Pads for Kids, Adults, Elderly, and Pets - Waterproof Protective Pad for Bed, Couch, Sofa, Floor

i bought some waterproof pads from amazon, they come in 4 packs and are roughly 3x3' squares. one should lay over the middle of a toddler mattress and be able to tuck in underneath so it doesn't get all bulky under the sheet. https://www.amazon.com/dp/B08SY6N9TL?ref_=ppx_hzsearch_conn_dt_b_fed_asin_title_6&th=1