You can buy 4 seats in economy plus and achieve what you are looking for. The bulkhead rows have fixed seat dividers, so don't book those, but any other row of 4 in the center grouping will work for you.

In addition, you can buy a couple of these pillows to fill the floor space so that your child has a larger flat area:

https://www.flyertalk.com/forum/28957738-post29.html

Buy it here:

https://1stclasskid.com/product/1st-...travel-pillow/