There are three plans. Pricing has the full comparison.
Free
Unlimited messages that can't be edited or deleted, a shared calendar, and your full history to view and print. It doesn't expire, and it covers both parents.
Family
Adds attachments and read receipts, the custody schedule builder, phone calendar sync, the evening summary, calls, expenses with receipts, children's information, and verified exports.
Family Pro
Adds the tone check, Message Shield, message search and filters, evidence packages, recorded calls with transcripts, video messages, GPS check-ins and handoff tracking, order import, receipt scanning, payment reminders and yearly summaries, and Moments.
Every plan covers both parents. There is no per-parent pricing and never a second bill.