J.S.
Two suggestions that I do myself:
1) Use vinegar regularly (in the detergent tray, softener tray or 'bleach' tray.) I use vinegar AS the softener and sometimes as the 'bleach'.
2) Use vinegar as a clean-out. Use the sanitize button or regular wash - no clothes - just vinegar.
Vinegar is a disinfectant, safe for little ones, will clean out the soap scum and mildew and keep the washer tip top.